ARCADIA-Studio/docs/language.rst

Arcadia Studio OA / SA Language and Editing Contract
====================================================

:Version: 4
:Status: Normative current implementation contract; first-release integration
:Core implementation: ``crates/oa-core``
:Text suffix: ``.arcadia``

1. Scope and conformance
------------------------

This specification defines the shared Operational Analysis (OA) and System
Analysis (SA) model, its textual language, graphical editing obligations,
validation, persistence, and explicit migration from versions 1, 2 and 3. MUST, MUST NOT,
and MAY indicate requirements, prohibitions, and permitted choices.

Schema 4 adds bounded reusable data types, typed scalar defaults, exchange
mechanisms, qualified message values, specialization, identified interface
allocations and reference associations. It retains schema 3 typed ports and
state views. Explicit older readers preserve existing meanings and identities.
This contract does not by itself establish native acceptance or release completion.

OA describes stakeholders, operational work and needs. SA describes the
selected system boundary, external actors, functions, services and expected
behavior. SA permits at most one System of Interest; its absence is an
incomplete-model warning when other SA elements exist. The SA System MUST NOT
contain internal components. Functional decomposition is independent of that
structural boundary. Logical Architecture, Physical Architecture, deployment,
Capella file interchange, and executable behavioral simulation are outside
this specification. This independent language does not claim complete Arcadia,
Eclipse Capella, or SysML conformance.

An implementation conforms only when each supported field and relationship is
reachable in both graphical and textual editing, preserves identity through
save/open and undo, and uses the same structural validation. Merely displaying
a name or storing an opaque payload does not establish graphical support.
Actual verification evidence is separate from this contract.

2. One authoritative model
--------------------------

A Project stores ``schema_version`` (4), ``id``, ``alias``, ``name``,
``description``, and maps named ``elements``, ``relations``, ``diagrams``,
``artifact_links``, and ``traces``. Each map is keyed by the UUID contained in
its record. Duplicate map keys and mismatching keys/identities are errors.

All objects share one UUID and alias namespace: Project, Element, Relation,
Diagram, NodePlacement, ArtifactLink, and Trace. UUIDs are durable identity;
aliases are durable, editable source labels. Display-name changes MUST NOT
regenerate existing aliases. Different objects MAY have the same display name.

Every Element has ``id``, ``alias``, ``name``, ``description``, ``parent``
(optional element reference), and a typed ``body``. Its kind and OA/SA layer
are derived from its body. A Relation has ``id``, ``alias``, ``name``,
``description``, and a typed body; it has no independently editable kind tag
that can disagree with its payload.

Diagram placement does not determine semantic ownership. NodePlacement
records belong to diagrams and reference semantic elements. Multiple diagrams
can show the same element with independent placement and collapse state.
Within one diagram, each element appears at most once.

All stored strings MUST contain only XML 1.0 characters: U+0009, U+000A,
U+000D, U+0020–U+D7FF, U+E000–U+FFFD, or U+10000–U+10FFFF. Strings are not
trimmed or Unicode-normalized. Escaping a forbidden character does not permit
it. Coordinates and sizes are finite IEEE-754 binary32 values with absolute
value at most 1000000. Width and height MUST be positive. Signed zero and
subnormal coordinates are retained.

3. Element declarations and typed fields
----------------------------------------

Each element declaration starts with its layer and kind, for example
``oa_activity`` or ``sa_function``. Every element body accepts
``description`` (default ``""``) and ``parent`` (default ``none`` for a
document-level declaration, or its lexical element owner when nested; section
6.2). The table
lists all additional fields. ``ref`` means ``@alias``; ``ref?`` also accepts
``none``. Lists preserve order. Unknown or duplicate fields are errors.

.. list-table:: Element body field catalog
   :header-rows: 1
   :widths: 27 43 30

   * - Declarations
     - Additional fields
     - Defaults
   * - ``oa_entity``, ``oa_actor``, ``oa_role``, ``oa_activity``, ``oa_capability``, ``oa_mission``
     - None
     - Empty body
   * - ``sa_system``, ``sa_actor``, ``sa_function``, ``sa_capability``, ``sa_mission``
     - None
     - Empty body
   * - ``oa_process``, ``sa_functional_chain``
     - ``steps``: occurrence refs
     - ``[]``
   * - ``oa_exchange_item``, ``sa_exchange_item``
     - ``kind``: Information, Material, Energy; ``mechanism``: ExchangeMechanism; ``supertype``: item ref?
     - Information; Unspecified; none
   * - ``oa_data_type``, ``sa_data_type``
     - ``definition``: DataTypeDefinition; ``default``: DataValue?; ``unit``: string
     - String with minimum_length 0 / maximum_length none; none; ``""``
   * - ``oa_data_field``, ``sa_data_field``
     - ``type_ref``; ``minimum``: u32; ``maximum``: optional u32; ``unit``: string; ``direction``: ParameterDirection; ``storage``: FieldStorage; ``default``: DataValue list?
     - String; 1; 1; ``""``; Unspecified; Value; none
   * - ``oa_interface``, ``sa_interface``
     - ``supertype``: interface ref?
     - none; membership is an identified interface_item_allocation relation
   * - ``oa_port``, ``sa_port``
     - ``contract``: required PortContract; ``legacy``: optional LegacyPortContract
     - No implicit contract; ``legacy`` defaults none. OA admits Legacy only.
   * - ``oa_scenario``, ``sa_scenario``
     - ``viewpoint``: ScenarioViewpoint; ``context``: ref?; ``precondition``, ``postcondition``: strings; ``steps``: occurrence refs
     - MixedLegacy for omitted schema-2 source; none; ``""``; ``""``; ``[]``
   * - ``oa_lifeline``, ``sa_lifeline``
     - ``represented``: participant, work, or role ref?
     - none
   * - ``oa_occurrence``, ``sa_occurrence``
     - ``body``: occurrence constructor; ``precondition``, ``postcondition``: strings
     - Exchange with none references, Asynchronous message kind and no values; two empty strings
   * - ``oa_state_machine``, ``sa_state_machine``, ``oa_region``, ``sa_region``
     - None
     - Empty body
   * - ``oa_state``, ``sa_state``, ``oa_mode``, ``sa_mode``, ``oa_initial``, ``sa_initial``, ``oa_final``, ``sa_final``
     - ``kind``: StateKind; ``entry``, ``exit``, ``do_activity``: strings
     - Kind selected by declaration; three empty strings
   * - ``oa_choice``, ``oa_junction``, ``oa_fork``, ``oa_join``, ``oa_shallow_history``, ``oa_deep_history``, ``oa_entry_point``, ``oa_exit_point``, ``oa_terminate`` and corresponding ``sa_`` declarations
     - Same State body; StateKind is Choice, Junction, Fork, Join, ShallowHistory, DeepHistory, EntryPoint, ExitPoint or Terminate
     - Kind selected by declaration; three empty strings
   * - ``oa_event``, ``sa_event``
     - ``body``: Signal, Time or Change constructor
     - Signal with ``items []``
   * - ``oa_constraint``, ``sa_constraint``
     - ``expression``: string; ``constrained``: semantic record refs
     - ``""``; ``[]``

The explicit state ``kind`` field MUST agree with its declaration keyword.
``u32`` is an integer from 0 through 4294967295. A DataField's maximum MUST
be at least its minimum; ``none`` means unbounded. Primitive TypeRefs are
Boolean, Integer, Real, and String. A named TypeRef uses a DataType reference.
An Enumeration contains unique string literals. For example:

.. code-block:: text

    definition { Enumeration { literals ["Clear", "Fog"]; }; };
    type_ref { Named @weather_classification; };

These are typed constructors with named fields, not embedded JSON documents.
Constructor field names and enum spellings are case-sensitive. When replacing
a constructor, all of its fields MUST be supplied; the containing declaration
defaults do not silently fill a different constructor's missing fields.
During schema-2 compatibility, the new Exchange fields ``message_kind``,
``reply_to`` and ``values`` are exceptions: omitted values hydrate as
Asynchronous, none and [] respectively. Canonical output writes them explicitly.
This preserves older schema-2 source/JSON without inventing call correlation or
value annotations. It is not a rule for an unspecified future schema.

.. list-table:: Occurrence body constructors
   :header-rows: 1
   :widths: 22 78

   * - Constructor
     - Required fields
   * - Exchange
     - ``exchange``: exchange relation ref?; ``sender``, ``receiver``: lifeline refs?; ``message_kind``: MessageKind; ``reply_to``: occurrence ref?; ``values``: MessageValue records
   * - Action
     - ``action``: activity/function ref?; ``lifeline``: ref?
   * - Control
     - ``kind``: Choice, Merge, Fork, Join, Loop
   * - SequenceReference
     - ``sequence``: process/functional-chain ref?
   * - ScenarioUse
     - ``scenario``: scenario ref?; ``bindings``: LifelineBinding records
   * - State
     - ``state``: state/mode ref?; ``lifeline``: ref?
   * - Fragment
     - ``operator``: Alternative, Parallel, Loop, Optional; ``branches``: branch occurrence refs
   * - Branch
     - ``guard``: string; ``occurrences``: occurrence refs

For example, ``body { Action { action @assess; lifeline @operator_line; }; };``
selects an Action occurrence. Processes and functional chains contain Exchange,
Action, Control and SequenceReference occurrences. Process/chain Actions are
contextual work contributions and MUST have ``lifeline none``. A Control or
SequenceReference MUST belong directly to a Process or FunctionalChain, not a
Scenario. Scenarios contain Exchange, Action, State, Fragment, Branch and ScenarioUse
occurrences according to their enclosing ownership rules. Each
listed occurrence belongs to its enclosing sequence or branch through
``parent``. A Branch belongs to a Fragment; the Fragment's ``branches`` list
references the branches it owns.

An exchange and an occurrence of that exchange have different identities.
Repeating an exchange requires distinct occurrence records. Repeating one
occurrence identity within an ordered list is invalid. Conditions, guards,
entry/exit/do text, and constraint expressions are descriptive strings; this
version does not parse or execute them as a programming language.

4. Containment, allocation, and relationships
---------------------------------------------

Containment MUST be acyclic and stay within one layer. Entities may contain
entities, actors, and roles. Actors are not structural containers. Activities
contain activities; functions contain functions. Capability grouping uses
same-layer capability parents; it does not mean inheritance or inclusion.
Operational and system missions are root semantic records; they do not own
capabilities by containment. MissionCapability expresses that association.
DataFields belong to ExchangeItems or Record DataTypes. Preserved generic ports belong to
participants or work. Lifelines belong to scenarios. State-machine ownership
uses participant → machine → region → state/mode; a state or mode can contain
a nested region. All other state vertices belong to Regions as well. Events
are root semantic records and do not acquire a machine owner when referenced.

DataFields, ports, lifelines, occurrences, Regions and all state vertices
require an owner. An unowned StateMachine can be saved as incomplete
work and produces a warning. The System cannot own internal components.
The present Studio profile does not decompose SA Actors.

All relations have a header identity, alias, display name, optional
``description`` (default ``""``), and required ``source`` and ``target``
references. Their kind is the declaration keyword:

.. list-table:: Relation catalog
   :header-rows: 1
   :widths: 24 49 27

   * - Declaration
     - Endpoint meaning
     - Additional fields / defaults
   * - ``allocation``
     - Same-layer participant → leaf activity/function
     - None
   * - ``interface_item_allocation``
     - Same-layer Interface → ExchangeItem
     - ``order`` u32 / 0; ``transmission`` TransmissionProtocol / Unspecified; ``acquisition`` AcquisitionProtocol / Unspecified
   * - ``data_association``
     - Same-layer Record DataType → Record DataType
     - ``source_role``, ``target_role`` / ""; ``source_minimum``, ``target_minimum`` / 0; ``source_maximum``, ``target_maximum`` / none; ``source_navigable``, ``target_navigable`` / false
   * - ``interaction``
     - OA activity → OA activity
     - ``items`` refs / []
   * - ``functional_exchange``
     - SA function → SA function
     - Required ``source_port``, ``target_port`` refs; ``items`` / []
   * - ``boundary_exchange``
     - Different participant owners; typed component ports have matching kinds
     - ``oriented`` bool / true; ``items`` refs / []
   * - ``port_allocation``
     - SA Component port → SA FunctionInput/FunctionOutput port
     - Requires the function's actual work Allocation to that component
   * - ``capability_involvement``
     - Participant, activity/function, or OA role → capability
     - None
   * - ``chain_involvement``
     - Process/functional chain → capability
     - None
   * - ``role_realization``
     - OA entity/actor → OA role
     - None
   * - ``role_assignment``
     - OA role → leaf OA activity
     - None
   * - ``mission_capability``
     - OA mission → OA capability, or SA mission → SA capability
     - None
   * - ``mission_involvement``
     - OA entity/actor → OA mission, or SA System/Actor → SA mission
     - None
   * - ``capability_include``
     - Including capability → included capability in one layer
     - None
   * - ``capability_extend``
     - Extending capability → extended capability in one layer
     - ``condition`` string / ""
   * - ``capability_generalization``
     - Specific capability → general capability in one layer
     - None
   * - ``sequence_link``
     - Preceding contribution → following contribution in one process/chain
     - ``guard`` string / ""
   * - ``state_transition``
     - State vertices in one machine, subject to section 4.4
     - ``trigger``, ``guard``, ``effect`` strings / ""; ``transition_kind`` External; ``triggers``, ``effects`` reference lists / []
   * - ``availability``
     - Work, participant, capability, or process/chain → state/mode
     - None
   * - ``exchange_allocation``
     - Boundary-exchange relation → SA functional-exchange relation
     - None
   * - ``interface_provision``, ``interface_requirement``
     - Participant/port → provided/required interface
     - None
   * - ``state_action``
     - State/mode → activity/function
     - ``phase``: Entry, Do, Exit / Do

Relation endpoints and carried ExchangeItems stay in the same layer. Parallel
exchanges with distinct identities are permitted. Self-relations are invalid
except for a state transition that returns to its source state or an explicit
DataAssociation between two ends of the same Record type.

Direct Allocation has at most one performer per leaf. Role sharing is a
different relationship: several OA participants can realize a Role. A direct
allocation inconsistent with its role's realized performers produces an
advisory warning; it does not prevent saving the model. Shared roles do not
imply a unique performer.
Allocation of a non-leaf work element is invalid. A computed graphical summary
of its allocated leaves MUST NOT create an extra semantic allocation.

Typed function ports belong directly to SA Functions. ``FunctionOutput``
declares ``outgoing_items``; ``FunctionInput`` declares ``incoming_items``.
The lists contain distinct same-layer ExchangeItem references. A functional
exchange MUST reference output/input ports owned by its respective functions,
and each carried item MUST fit both declarations. Empty declarations accept
no items. Item compatibility is identity equality; future specialization is
not inferred from names. Port declarations remain independent of incident
exchanges after creation.

A ``Component`` contract belongs to an SA System or SystemActor and stores
``kind`` (Standard or Flow) and ``orientation`` (Unset, In, Out, InOut).
Standard requires Unset. Flow Unset is an incomplete draft, not InOut. Typed
component connections require matching kinds: Standard/Flow mismatch is an
error. Standard connections normally use ``oriented false``; directional Flow
connections use true. Boundary orientation is separate from endpoint kind and
orientation. Oriented Flow endpoints permit Out at source and In at target;
unoriented Flow endpoints must permit at least one communication direction.

Provided/required interfaces are separate identified relations. A Standard
partner satisfies a requirement by providing the identical interface; stored
source/target order does not infer service roles. Component carried items must
fit the declared interface-item union when explicit declarations exist. An
explicitly empty interface is not a wildcard. Missing declarations and
unsatisfied required interfaces produce located completeness warnings.

``PortAllocation`` relates a Component port to a typed function port only when
that function has an actual Allocation to the component owner. Standard may
allocate either function direction; Flow directions must agree. A typed
ExchangeAllocation requires both corresponding PortAllocations and compatible
carried items. Unoriented boundary exchanges allow reversed correspondence.
Visual allocation nesting is never evidence of a stored allocation relation.

``Legacy`` requires ``legacy { direction ...; interface ...; }``. It preserves
generic direction/interface data and emits a located compatibility warning.
It is an explicit compatibility constructor in JSON/source; ordinary creation
pickers do not offer it. Resolution to a typed SA contract retains this record
without treating it as another active constraint. OA generic ports remain
Legacy; OA Interactions stay activity-to-activity without function ports.
Deleting a referenced legacy interface clears that optional live reference.

A source-3 functional exchange MUST specify both port aliases; the parser does
not create endpoints. Graphical/command FE creation plans ports atomically,
using unique compatible typed-port reuse or deterministic new endpoints.
Port deletion cascades to incident FEs; FE deletion retains independent ports.

For example::

    arcadia 4;
    project ports_example "Typed function contract" {
        sa_function send "Send" {}
        sa_function receive "Receive" {}
        sa_exchange_item payload "Payload" {}
        sa_interface old_service "Preserved service" {}
        interface_item_allocation service_payload "Service payload" { source @old_service; target @payload; }
        sa_port output "Output" {
            parent @send;
            contract { FunctionOutput { outgoing_items [@payload]; }; };
        }
        sa_port input "Input" {
            parent @receive;
            contract { FunctionInput { incoming_items [@payload]; }; };
        }
        functional_exchange message "Message" {
            source @send; target @receive;
            source_port @output; target_port @input; items [@payload];
        }
        sa_port preserved "Unresolved original" {
            parent @send; contract Legacy;
            legacy { direction InOut; interface @old_service; };
        }

An exclusive region has at most one initial state. Missing initial state is
a completeness warning. Initial states have no incoming transitions and at
most one outgoing transition; their outgoing transition has no trigger/guard.
Final states have no outgoing transitions. Mixing State and Mode entries is
permitted by this Studio profile. StateAction links provide typed work
references alongside the descriptive entry/exit/do strings.

4.1. Capabilities and missions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Include means the including capability requires the included capability's
behavior. Extend means the source capability adds conditional behavior to the
target; its ``condition`` records when that behavior applies. Generalization
means a specific source capability specializes a general target capability.
Each relationship stays within OA capabilities or within SA capabilities.
Include and Generalization graphs MUST each be acyclic, including transitive
cycles. Extend does not introduce an additional cycle restriction; the ordinary
no-self rule still applies. An empty Extend condition is an engineering warning
so an incomplete capability map can be authored and saved.

MissionCapability and MissionInvolvement express need and stakeholder/system
participation. They MUST NOT create a capability parent, a performer allocation,
or a cross-layer realization. These relationships remain distinct when the
same objects appear in a diagram or share names.

4.2. Contextual process and chain topology
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A Process or FunctionalChain owns contribution occurrences with stable
identities. ``steps`` is their persistent membership and presentation order;
an identity MUST occur at most once in that list. A work contribution points to
an existing same-layer Activity or Function. Reusing the same work or exchange
requires separate occurrence identities and MUST NOT duplicate the referenced
engineering element. A SequenceReference points to another same-layer Process
or FunctionalChain of the owner's kind. Its target may be ``none`` while the
draft is incomplete. Direct and transitive recursive sequence reuse are errors.

SequenceLinks connect distinct occurrences directly owned by the same process
or chain. Their source is the predecessor and target is the successor. A link
from work to an exchange requires that work to be the referenced exchange's
source; an exchange-to-work link requires the target work to be its endpoint.
Two adjacent exchange contributions require the first exchange's target to
match the second exchange's source. A mismatch is a structural error. Other
links express contextual precedence without manufacturing a global exchange.

When links or work/control/reuse contributions exist, topology is explicit.
Reordering ``steps`` MUST NOT add, remove or reorder SequenceLinks. The editor
MUST retain the difference between alternative Choice/Merge and parallel
Fork/Join contributions. Choice/Fork expect one incoming and at least two
outgoing links; Merge/Join expect at least two incoming and one outgoing link.
Loop expects at least one incoming and two outgoing links for repetition and
exit. These cardinalities are completeness warnings, not barriers to building
an incomplete flow. Choice and Loop outgoing paths with empty guards warn;
an explicit ``else`` guard is permitted. Guards remain descriptive strings.

Validation warns about missing/ambiguous entry and exit, isolated or unreachable
contributions, contributions with no reachable exit, and splits/merges that
lack explicit controls. Topology cycles are permitted for loops; the recursive
SequenceReference prohibition is a different rule. A legacy exchange-only
sequence without explicit links retains its ordered linear meaning and
adjacent-exchange gap diagnostics. Implementations MUST NOT silently invent
topology links when loading or reordering such a sequence.

4.3. Typed scenarios and reusable behavior
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

ScenarioViewpoint is one of OperationalActivity (OA activity lifelines),
ParticipantInteraction (OA entity/actor/role lifelines), Functional (SA function
lifelines), BoundaryInterface (SA System/Actor lifelines), or MixedLegacy.
The first two are OA viewpoints; the next two are SA viewpoints. Lifeline
references MUST agree with that perspective. OperationalActivity messages use
Interactions; Functional messages use FunctionalExchanges. Participant views
may use their layer's work exchanges through effective performer allocation,
or participant boundary exchanges through the relevant ports. Corresponding
lifelines and endpoints MUST agree; a viewpoint is not a label that suppresses
reference checks.

Omitted schema-2 viewpoints hydrate as MixedLegacy in both source and JSON.
This preserves pre-extension mixed work/participant scenes without discarding
their lifelines or silently assigning a stricter meaning. MixedLegacy emits a
review warning. New graphical factories explicitly select OperationalActivity
or Functional by layer. Authors can select a strict viewpoint after reviewing
the legacy scene; canonical source then records that choice.

MessageKind is Asynchronous, Call, Reply, Create, Delete, or
``{ Timer { expression "after 5 s"; }; }``. A process/chain exchange
contribution MUST remain Asynchronous with no reply correlation. Call/reply,
creation/deletion and timer semantics belong to scenarios. Reply's ``reply_to``
references a Call occurrence in the same scenario. The call MUST precede the
reply on a guaranteed common sequence/enclosing branch path. Reply lifelines
reverse the call's sender/receiver; it may reuse the call's exchange in reverse
or use an explicit reverse-direction exchange. A second reply to the same call
on that guaranteed path is an error. Missing reply correlation is a draft
warning; non-Reply messages MUST NOT have ``reply_to``.

Create identifies the receiver lifeline's creation. An earlier guaranteed use
of that lifeline is an error. Delete identifies its destruction; a later
guaranteed use is an error. Ordering checks respect branch ownership; they MUST
NOT flatten alternative branches into a false unconditional order. A Timer is
local to its sender lifeline, has no global exchange or values, and has either
no receiver or the same receiver as sender. Its nonempty expression describes
the delay/deadline; an empty expression warns. Conditions and expressions are
stored engineering annotations, not an executable simulation language.

Each MessageValue is ``{ path [@parameter]; field @data_field; expression "value expression"; }``.
Its identity is the pair (path, field), so two uses of one Record can have
different values for the same terminal field. Path contains enclosing DataField
references, excluding the terminal field; direct parameters use ``[]``. Every
hop MUST follow Value storage through a named Record's effective fields. Reference
storage stops traversal. Operation Call accepts In/InOut and Reply accepts
Out/InOut/Return, interpreted from the provider's perspective. Nested field paths
inherit their direct parameter's direction. Unspecified direction warns.
At most one annotation for a pair is allowed. Omitted path defaults to ``[]``;
an ambiguous preserved legacy nested annotation warns ``message_value_ambiguous_path``
until an explicit path is selected. New graphical choices always identify a path.
Values never overwrite types or items. Expression text is descriptive, preserved
exactly and not evaluated; an empty expression warns.

ScenarioUse references an existing same-layer scenario and maps its lifelines
with ``bindings [ { referenced @callee_line; local @caller_line; } ];``.
Each binding is one-to-one and references a direct lifeline of the appropriate
scenario. Bound lifelines represent the same semantic object, or an explicitly
realized operational role. Strict viewpoints MUST agree; MixedLegacy retains
the compatibility case until reviewed. Missing bindings produce warnings.
Direct/transitive recursive scenario use is invalid; repetition uses a Loop
fragment instead. ScenarioUse owns neither its referenced scenario nor its
lifelines; deleting a use does not delete the reusable definition.

4.4. State vertices, events and transition contracts
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

StateKind comprises State, Mode, Initial, Final, Choice, Junction, Fork, Join,
ShallowHistory, DeepHistory, EntryPoint, ExitPoint and Terminate. Every vertex
belongs to a Region. Only State and Mode are stable states: only these may own
nested Regions, typed StateAction links, availability and scenario state
observations. Multiple sibling Regions describe orthogonal configurations.
EntryPoint and ExitPoint retain Region ownership; their boundary is that
Region's owning State/Mode or StateMachine. Their graphical symbols belong on
that boundary. This is Studio's explicit containment encoding.

Events are ordinary same-layer semantic elements, independent of diagram
placements. EventBody has exactly these constructors:

.. list-table:: Event definitions
   :header-rows: 1

   * - Constructor
     - Fields and meaning
   * - Signal
     - ``items`` is a unique list of same-layer ExchangeItem references; empty is valid
   * - Time
     - ``kind`` is After or At; ``expression`` is descriptive time text
   * - Change
     - ``expression`` describes the Boolean condition becoming true

After measures relative time from activation of a transition's source state;
At describes an absolute instant. Empty Time/Change expressions are located
completeness warnings. The application does not execute expressions, fire
timers or transport signals. For example::

    sa_event timeout "Recovery timeout" {
        body { Time { kind After; expression "30 s"; }; };
    }
    sa_event recovered "Pressure recovered" {
        body { Change { expression "pressure > 4 && status == \"Ready\""; }; };
    }

StateTransition's ``triggers`` references Events or layer-appropriate work
(OA Activity / SA Function). Referenced work need not have the machine owner's
performer: external work can cause the observed transition. Multiple triggers
are alternatives. ``effects`` references work describing effects, without
executing it. Duplicate, missing, wrong-layer or wrong-kind references are
errors. The existing ``trigger``, ``guard`` and ``effect`` strings remain
authored prose; they MUST NOT be renamed, parsed as UUIDs or inferred by name.

With typed triggers, ``trigger`` adds descriptive information. With no typed
triggers and nonblank trigger prose, the transition has an unresolved
descriptive trigger and emits a warning. Both empty denotes completion from
a stable state or transient routing from a pseudostate. Deleting the final
typed trigger when prose is empty MUST leave an explicit descriptive reminder;
it MUST NOT silently create a completion transition. Guards/effect prose remain
unchanged. Signal payload references and typed effect references are pruned
when their targets are removed.

TransitionKind is External, Local or Internal. External leaves/re-enters the
affected configuration; a stable self-transition includes exit and entry
behavior. Internal requires the same stable source and target, preserves nested
configuration, and does not imply entry/exit. Local requires a composite stable
source and proper descendant target, preserving the source activation; an
EntryPoint also routes locally into its boundary. Local self/outside targets
are invalid. Other pseudostates route externally. Endpoints MUST share a layer
and StateMachine; crossing nested Regions within that machine is not generally
forbidden. A pseudostate self-transition is invalid.

A simple state's completion means entry/do behavior is complete. A composite
completes when its active Regions complete. Final completes its containing
Region; Terminate ends the containing machine's activation. These are modeling
meanings, not evidence of a running state-machine engine.

Missing connections/guards and unfinished initial/history paths produce
warnings, so an engineer can insert vertices before wiring them. Contradictory
upper cardinalities, forbidden directions/kinds and invalid references reject.

.. list-table:: Vertex connection constraints
   :header-rows: 1

   * - Vertex
     - Structural restrictions
     - Incomplete-model warnings
   * - Initial
     - At most one per Region; no incoming; at most one outgoing; no outgoing trigger or guard
     - Missing initial/default entry or outgoing
   * - Final / Terminate
     - No outgoing or nested Region
     - No incoming
   * - Choice
     - At most one incoming; outgoing transient alternatives
     - Missing incoming, fewer than two outgoing, missing guard or else
   * - Junction
     - Transient routing, no implicit concurrency
     - Missing side or ambiguous unguarded alternatives
   * - Fork
     - At most one incoming; outgoing stable targets in distinct orthogonal Regions
     - Missing incoming or fewer than two outgoing
   * - Join
     - At most one outgoing; incoming stable sources in distinct orthogonal Regions
     - Fewer than two incoming or missing outgoing
   * - ShallowHistory
     - At most one per Region; at most one fallback to a direct stable child of that Region; no fallback trigger/guard
     - Missing fallback/default entry
   * - DeepHistory
     - At most one per Region; at most one fallback to a stable descendant of that Region; no fallback trigger/guard
     - Missing fallback/default entry
   * - EntryPoint
     - Incoming from outside boundary, outgoing Local into it; at most one outgoing per destination Region
     - Missing outgoing or incomplete affected-Region entry
   * - ExitPoint
     - Incoming from inside boundary; at most one outgoing External to outside
     - Missing input/output; top-level exit is an exit endpoint, not implicit termination

Fork outputs and Join inputs MUST NOT carry guards or typed/descriptive
triggers. Their distinct orthogonal Regions are the first Regions beneath the
nearest common composite or machine, not merely each endpoint's immediate
parent. Choice/Junction routing expresses conditional alternatives. Retained
pseudostate entry/exit/do prose produces a warning and remains editable; it
does not become a typed state behavior. StateAction remains the maintained
reference for entry/do/exit work on stable states.

Other pseudostate outgoing segments MUST NOT introduce event/work triggers
or trigger prose. A Choice has at most one literal ``else`` guard; the tool
does not prove arbitrary guard expressions disjoint or exhaustive. Choice
expresses a dynamic decision after incoming effects; Junction expresses a
compound path selected by guards before traversing it. ShallowHistory remembers
the immediate state and uses default entry for deeper configuration;
DeepHistory remembers nested configuration. Fallback applies when no resumable
history exists. History is per Region, including top-level Regions in this
Studio profile; no running history state is persisted or synthesized.

4.5. Data contracts, specialization and interface allocation
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

ExchangeItem ``kind`` describes nature: Information, Material or Energy.
``mechanism`` is Unspecified, Flow, Event, Operation or SharedData. Unknown
mechanisms remain explicit Unspecified drafts with a warning. Material/Energy
with Event/Operation/SharedData warns for engineering review; it is not silently
reclassified. Empty Event notifications and void Operations are valid. An empty
carried/allocated Flow, SharedData or Unspecified item warns.

An Operation's directly owned DataFields are parameters. ``direction`` is
Unspecified, In, Out, InOut or Return, from the provider's perspective. At most
one effective parameter is Return. Unspecified parameters warn; non-operation
fields MUST use Unspecified. Nested record fields inherit direction from their
direct parameter when used in message paths, rather than storing context on a
shared type. ``storage`` is Value or Reference. Reference requires a named Record,
has no explicit default, and does not convey ownership or deletion of the type.

DataTypeDefinition has exactly the following constructors:

.. list-table:: Reusable definitions
   :header-rows: 1

   * - Constructor
     - Required fields
   * - Boolean
     - None
   * - Integer
     - ``minimum``, ``maximum``: optional signed 64-bit integers
   * - Real
     - ``minimum``, ``maximum``: optional finite IEEE-754 binary64 values
   * - String
     - ``minimum_length``: u32; ``maximum_length``: optional u32
   * - Enumeration
     - ``literals``: unique nonempty, case-sensitive strings
   * - Record
     - ``supertype``: same-layer Record ref?

Bounds are inclusive and ordered; an absent bound means unbounded within the
represented numeric domain. Integer comparisons MUST NOT convert to binary64.
NaN, infinities, overflow and reversed bounds reject. String length counts Unicode
scalar values, not UTF-8 bytes or grapheme clusters. Enum defaults match exact
literals. Empty enum definitions remain incomplete drafts with a warning.

DataValue is a tagged scalar: ``{ Boolean true; }``, ``{ Integer 42; }``,
``{ Real 1.5; }``, ``{ String "value"; }`` or ``{ Enumeration "Ready"; }``.
There is no implicit numeric or string coercion. A type default is one compatible
scalar within its bounds. Record type defaults and record/object instance literals
are unsupported. A field default is absent (``none``) or a list of compatible
scalars with length inside its multiplicity. ``[]`` explicitly supplies an empty
collection and requires minimum 0, including Value Record fields; a nonempty
Record literal rejects. Reference fields MUST use ``none``. A one-valued scalar
field uses a one-value list. A named type default supplies an individual element's
default, not a collection or an implicit instance of a mandatory field.

Numeric types may carry unit text. Empty field units inherit the named type unit;
an explicit different unit warns when that type has a unit. Units on nonnumeric
fields/types warn and remain preserved. Unit strings never convert values or
establish dimensional compatibility. Expressions are not executed.

Record, ExchangeItem and Interface each permit one same-family, same-layer
supertype. Specialization is acyclic and separate from containment. Effective
fields are base fields followed by local fields, preserving original field UUIDs
and owners. Local names MUST NOT shadow inherited names. Item specialization
preserves base nature and any specified mechanism; an Unspecified base mechanism
may be refined. Deleting a derived record never deletes its base. Mandatory
Value-field cycles warn ``record_no_finite_value``; optional or Reference cycles
are valid without that warning. Traversal is cycle guarded and bounded; a payload
path budget failure is an explicit diagnostic, not silent truncation.

InterfaceItemAllocation is an ordinary identified relation from Interface to
ExchangeItem. It stores ``order`` and independent transmission/acquisition
protocols. Transmission is Unspecified, Synchronous, Asynchronous, Unicast,
Multicast or Broadcast; acquisition is Unspecified, Read or Accept. No protocol
combination is inferred from mechanism. Effective allocations are base-first,
then local ordered by (order, alias, UUID). A local item already allocated locally
or inherited is an error. Removing an allocation keeps its item and other
allocations; if live boundary compatibility would break, the transaction MUST
repair the affected contract explicitly or reject atomically. Artifacts can
reference the allocation identity independently of the global item.

An exchange carrying a specialized item satisfies a declaration of its base,
never the reverse. Standard provided/required compatibility uses explicit
interface ancestry. Identity/ancestry, not matching names, defines conformance.
The current typed function/component port contracts remain unchanged.

DataAssociation connects Record types without adding fields or transitively
carrying payload. Each named end has role, multiplicity and navigability. Source
multiplicity counts source instances per target instance and vice versa; it is
not a count of model elements. An end's navigable flag permits navigation toward
that end. Self-associations are valid; if both ends are navigable they require
distinct nonempty roles. Empty roles and duplicate role contracts warn, whereas
invalid ranges reject. Composition/shared aggregation, multiple inheritance,
redefinition, instance literals and arbitrary-precision decimal arithmetic are
outside this bounded contract.

This complete data contract example is also exercised through the REST API:

.. code-block:: text

    arcadia 4;
    project engineering_data "Engineering data" {
        sa_data_type voltage "Voltage" {
            definition { Real { minimum 0.0; maximum 48.0; }; };
            default { Real 24.0; }; unit "V";
        }
        sa_data_type coordinate "Coordinate" {
            definition { Record { supertype none; }; };
            sa_data_field latitude "Latitude" { type_ref Real; }
        }
        sa_data_type located "Located asset" {
            definition { Record { supertype @coordinate; }; };
            sa_data_field supply "Supply" { type_ref { Named @voltage; }; }
        }
        sa_exchange_item navigate "Navigate" {
            mechanism Operation;
            sa_data_field origin "Origin" { type_ref { Named @coordinate; }; direction In; }
            sa_data_field destination "Destination" { type_ref { Named @coordinate; }; direction In; }
            sa_data_field accepted "Accepted" { type_ref Boolean; direction Return; }
        }
        sa_interface navigation "Navigation" {}
        sa_interface monitoring "Monitoring" {}
        interface_item_allocation nav_operation "Navigation operation" {
            source @navigation; target @navigate; transmission Synchronous; acquisition Accept;
        }
        interface_item_allocation monitor_operation "Monitoring operation" {
            source @monitoring; target @navigate; transmission Asynchronous; acquisition Read;
        }
        data_association location_link "Location relationship" {
            source @coordinate; target @located;
            source_role "location"; target_role "assets";
            source_maximum 1;
        }
    }

5. Diagrams, artifacts, and cross-layer traceability
----------------------------------------------------

``diagram`` has name, identity, alias, description, ``layer`` (Oa by default),
``projection`` (Overview by default), and ``routes`` (empty list by default).
The string projections are Overview, Decomposition, Allocation,
FunctionalDataflow, Capabilities, and Traceability. StateMachine is a payload
constructor with one required ``machine`` reference. Traceability diagrams
can show both layers; other diagrams show their chosen layer.

Allocation is an architecture view: it shows participants and work, nests
allocated work under its performer, and displays interactions, functional
exchanges and boundary exchanges. FunctionalDataflow shows the work hierarchy
and its exchanges. Capabilities shows participants, roles, missions and
capabilities with their involvement, exploitation, include, extend and
generalization links. Decomposition and Overview retain general containment;
Overview also exposes all relationships. These string projections hide
inapplicable placements without deleting them or changing semantic records.

A StateMachine projection MUST reference an existing StateMachine in the
diagram's layer. Its placements MUST refer to that machine, its Regions, or
its state vertices, including nested Regions and vertices. Events, other
machines and their members, and unrelated semantic elements are not valid
placements in this view. Empty and partial state views are valid; neither
creating nor reading a view synthesizes semantic elements or placements.
Changing an existing diagram to StateMachine, or changing its machine, MUST
remove or relocate incompatible placements in the same transaction. Context
errors use ``state_diagram_context``; incompatible placements use
``state_diagram_member``. Neither case silently hides invalid members.

This complete source example places one state from an existing machine:

.. code-block:: text

    arcadia 4;
    project operating "Operating modes" {
        oa_entity crew "Crew" {}
        oa_state_machine modes "Modes" { parent @crew; }
        oa_region main "Main region" { parent @modes; }
        oa_state ready "Ready" { parent @main; }
        diagram mode_view "Operating modes" {
            layer Oa;
            projection { StateMachine { machine @modes; }; };
        }
        node ready_view { diagram @mode_view; element @ready; }
    }

The JSON projection is
``{"StateMachine":{"machine":"11111111-1111-4111-8111-111111111111"}}``,
where the UUID is the referenced machine's identity. A bare ``"StateMachine"``,
a missing or null ``machine``, an invalid UUID, or additional payload fields
is malformed. Existing projections retain their string representation, for
example ``"Overview"`` in JSON and ``projection Overview;`` in source.

Ports are boundary anchors of visible owners, with their existing
semantic identity. An explicitly placed port is not rendered as a separate
large node. Functional exchanges use their declared source/target ports;
boundary exchanges attach to their participant ports even if those ports have
no separate placement. Collapsing an owner hides its interior; projected links
may terminate at a visible ancestor without creating a new port or exchange.

``node`` has an alias and identity but no display-name string. Required fields
are ``diagram`` and ``element`` references. Optional fields are ``position``
(record with x/y, default 0/0), ``size`` (width/height, default 224/94), and
``collapsed`` (false), and ``port_anchors`` (empty list). Node identity is
independent of its semantic element.
Coordinates are absolute diagram coordinates. Placement does not alter parent,
allocation, or realization. Hiding an element from a diagram must remain
distinguishable from deleting it from the model.
Changing a node's diagram preserves the placement identity, alias, coordinates,
size, collapse state, manual port anchors, and attached artifacts. The destination
must accept the element's layer, satisfy StateMachine membership when applicable,
and must not already contain another placement of that element.

Each ``port_anchors`` entry is a PortAnchor record with required ``port``
reference, ``side`` (Left, Right, Top, or Bottom), and ``offset`` (finite
binary32 from 0 through 1). It has no separate identity or alias. The port MUST
be an existing Port directly owned by the placement's ``element``. A port MAY
have one manual anchor per placement; different diagrams MAY position it
independently. List order is preserved but has no positional meaning.

``offset`` measures a fraction of the complete projected owner edge, from top
on Left/Right or from left on Top/Bottom. Zero and one denote its corners.
The side is presentation data and MUST NOT change the port's contract,
orientation, ownership, allocations, or connected exchanges. Absent anchors use
automatic positioning. Empty lists are omitted in canonical JSON/source;
earlier schema-3 input without this field hydrates as ``[]``.

.. code-block:: text

    node actor_view {
        diagram @architecture; element @vessel;
        port_anchors [{ port @communication; side Top; offset 0.25; }];
    }

``SetPortAnchor { diagram, node, anchor }`` upserts by port identity and retains
unrelated entries. ``RemovePortAnchor { diagram, node, port }`` restores
automatic positioning; it is a no-op only for an existing direct child port
of that placement. Unknown fields, duplicate port references, non-port or
foreign-owner references, and invalid offsets are structural errors. Each
anchor counts as one record against ``ParseLimits.max_records``.

These commands use the same atomic history, conditional REST transactions, and
source writeback as other presentation edits. Source references resolve across
files, and anchor comments follow port identity. Reidentifying a port updates
anchor references; deleting it removes its anchors. Reparenting MUST repair
old-owner anchors in the same transaction, for example by removing them before
the Reparent command. Moving, resizing, arranging, or folding an owner retains
its authored sides and fractions. Frozen schema-1/schema-2 readers do not accept
``port_anchors``; their explicit migration supplies an empty list.

Each ``routes`` entry is an EdgeRoute record with a required ``edge`` reference
to an existing Relation or Trace, ``waypoints`` (default ``[]``), and ``label``
(default ``none``). Route records have no separate UUID or alias. An edge occurs
at most once in a diagram's route list; different diagrams MAY store independent
routes for the same edge. List order and explicit empty entries are retained by
source and JSON round trips, although list order has no routing meaning. Edges
need not currently be visible in their diagram.

.. code-block:: text

    routes [
        { edge @report_exchange;
          waypoints [{ x 420; y 260; }, { x 690; y 260; }];
          label { fraction 0.55; offset { x 0; y -16; }; };
        }
    ];

Waypoints are pinned absolute diagram positions in source-to-target traversal
order; at most 128 are allowed per edge. Computed endpoints and automatic elbows
MUST NOT be persisted as authored waypoints. A label record requires both
``fraction`` (finite binary32 from 0 through 1, measured along the computed
polyline's arc length) and ``offset`` (a relative x/y diagram displacement).
All position/displacement components obey the coordinate bounds in section 2.
An absent label uses automatic placement. Unknown fields, missing required
nested fields, duplicate route edges, non-edge references and invalid numbers
are structural errors. A pinned point that obstructs routing remains stored;
geometric routing difficulty is reported separately from structural validity.
Each EdgeRoute counts as one record against ``ParseLimits.max_records``; pinned
points are bounded by the per-route limit and aggregate input byte limits.

``SetEdgeRoute { diagram, route }`` upserts an entry by edge identity, retaining
the order and contents of unrelated entries. ``RemoveEdgeRoute { diagram, edge }``
removes it; removing an absent entry is a no-op for an existing diagram and edge.
The latter is the Reset routing operation. Either command participates in the
same atomic transactions, validation and undo as other model edits. Deleting a
relation or trace removes its routes in every diagram; reidentifying it updates
route references without changing geometry. Folding, hiding, changing projection
or relocating placements MUST NOT erase routing metadata. Group movement
translates pins when both actual endpoint owners move by the same delta; label
offsets remain relative. Raw ``EditNode`` and source position edits are exact
record edits and do not implicitly translate pins.

This is an additive schema-3 field. Earlier schema-3 JSON/source that omits
``routes`` loads as an empty list. Frozen schema-1 and schema-2 readers do not
accept this new field; migration creates empty route lists. Source projects keep
routes in their owning diagram declaration/file, resolve ``edge @alias`` across
files, and retain route-entry comments by referenced edge identity when entries
are reordered. The existing REST transaction endpoint accepts the same externally
tagged command records with UUID references and conditional ETags; no separate
routing endpoint or UI-only routing model exists.

``artifact`` has name, identity, alias, description, required ``owner`` ref,
``relation`` (References, Satisfies, Verifies, Implements, DerivedFrom;
default References), ``uri`` (default empty), ``revision`` (optional string),
and ``fragment`` (optional string). The two optional strings default to none.
URI must be nonempty, have no whitespace/backslash/control characters, and
use valid percent escapes. The owner is another existing model object.
Validation MUST NOT fetch the URI. Revision and fragment are retained verbatim.

``trace`` has name, identity, alias, description, required ``oa`` and ``sa``
endpoint references, ``kind`` (default Refinement), and ``baseline`` (default
none). Endpoints are canonical: SA realizes OA, regardless of which side
initiated creation. Trace does not change endpoint identity or containment.
Multiple distinct counterparts are permitted; duplicate endpoint/kind triples
are invalid.

.. list-table:: Trace kinds
   :header-rows: 1
   :widths: 23 77

   * - Kind
     - OA → SA endpoint types
   * - Refinement
     - Activity → Function
   * - Participant
     - Entity/Actor → System/Actor
   * - Capability
     - Operational Capability → System Capability
   * - Mission
     - Operational Mission → System Mission
   * - Exchange
     - Interaction relation → FunctionalExchange relation
   * - Sequence
     - Process → FunctionalChain, or paired occurrence contributions directly owned by Process / FunctionalChain
   * - Item
     - Same shared kind: ExchangeItem, DataType, DataField, Interface, or Port
   * - Scenario
     - Same shared kind: Scenario, Lifeline, or Occurrence
   * - State
     - Same shared kind: StateMachine, Region, any StateKind vertex, or Event
   * - Manual
     - Engineer-defined relationship between OA/SA elements or relations

A nonempty baseline is ``{ oa { name "..."; description "..."; };
sa { name "..."; description "..."; }; }``. It records last-reviewed names
and descriptions, not a full structural snapshot. Implementations MUST NOT
present it as structural synchronization or a lossless inverse transition.
Explicit counterpart creation, linking, comparison, and reviewed property
update may start in either layer. Ordinary edits MUST NOT automatically
overwrite the counterpart. Deletion MUST NOT silently cascade across layers.

Operational Capability normally creates a System Capability. Operational Mission
and System Mission are default counterparts in both directions and use Mission
traces. An explicit Operational Capability ↔ System Mission mapping remains
supported with a Manual trace; a uniquely linked such counterpart is reused
instead of silently creating a different mapping. Required/subtree process and
chain transitions include their contextual links, owned contributions and
referenced dependencies. New process/chain contribution mappings use Sequence
traces; older Scenario-kind occurrence mappings remain accepted. The current
name/description baseline does not reconcile later topology changes.

Name and description are compared independently against each endpoint's
last-reviewed baseline. An unchanged source MUST NOT overwrite an accepted
different target value. A source-only change proposes an update; different
changes on both sides require an explicit decision to take the source, keep
the target, or enter a merged value. An explicit decision records both current
values as reviewed, even when they differ. A target-only edit is preserved
without advancing that field's baseline, so it remains eligible to propagate
in the reverse direction. Equal current values can be recorded as converged.

6. Text grammar and resolution
------------------------------

The input is UTF-8 without a byte-order mark. LF, CR, space, and tab separate
tokens. ``//`` starts a comment through the next LF or end of input. Quoted
strings use JSON string escaping, including ``\n``, ``\r``, ``\t``, ``\\``,
and Unicode escapes. Literal unescaped newlines inside strings are invalid.
Numbers use JSON decimal/exponent syntax; NaN, infinity, hexadecimal numbers,
leading plus signs, and leading zeroes are invalid.

Aliases match ``[A-Za-z_][A-Za-z0-9_]*``. The following aliases are reserved:
arcadia, project, id, oa, sa, element, relation, diagram, node, artifact,
trace, body, alias, name, description, parent, from, to, entity, actor,
activity, capability, process, allocation, interaction, contribution,
realization, position, steps, true, false, null. Other field/constructor
spellings can be used in alias positions because references begin with ``@``.

The grammar below uses ``*`` for repetition and ``?`` for optional syntax.
``element-kind`` and ``relation-kind`` are exactly the corresponding declaration
keywords listed above. Typed field
sets and enum constructors are closed by sections 3–5.

.. code-block:: text

    document    = "arcadia" "4" ";" project
    project     = "project" alias string identity? "{" project-entry* "}"
    project-entry = "description" string ";" | declaration
    declaration = element
                | (relation-kind | "diagram" | "artifact" | "trace")
                  alias string identity? fields
                | "node" alias identity? fields
    element     = element-kind alias string identity? element-body
    element-body = "{" (field-name value ";" | element)* "}"
    identity    = "id" uuid-string
    fields      = "{" (field-name value ";")* "}"
    value       = string | number | "true" | "false" | "none"
                | enum-word | "@" alias | list | fields
    list        = "[" (value ("," value)*)? "]"

The project description defaults to empty and may appear once. There is no
semicolon after a declaration or project closing brace. Every field inside a
record ends with a semicolon, including fields whose values are nested records.
Trailing list commas are invalid. Fields may appear in any order.

UUID strings use 8-4-4-4-12 hexadecimal spelling, with uppercase or lowercase
hexadecimal accepted. Canonical output uses lowercase. Omitting an identity
creates a fresh UUID for that parse. After Apply, accepted source includes the
generated identity. Authors must keep identity clauses to preserve existing
objects across subsequent edits.

All declaration aliases are collected before references resolve, so forward
references are legal, including references to nodes and diagrams. Unknown
``@alias`` values are errors at their reference occurrence. Quoted UUIDs and
bare words cannot substitute for reference-valued fields. Alias resolution
produces UUID references; the alias itself remains stored on its declaration.
Changing an alias in source requires updating its uses. Display names never
resolve references.

Unknown fields, duplicate fields, incompatible constructor payloads, unsupported
enum spellings, duplicate aliases/identities, and structural validation errors
reject the entire candidate. No declaration silently replaces another.
Ordinary string fields require quoted strings and cannot contain an unquoted
``@alias`` reference. Enum fields require unquoted named values or their typed
constructor records. Typed values MUST NOT nest more than 64 levels; malformed
deeply nested drafts produce a diagnostic instead of exhausting the parser stack.

6.1. Modular source projects
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The pure ``source_project`` core API compiles a virtual source bundle, containing
``entry`` and a ``files`` map from logical paths to UTF-8 strings. Its entry is a
root-level ``.arcadia`` file with the source4 project header above. A project entry
MAY also be ``include string ";"``. An included fragment consists only of ordinary
declarations and include statements; it MUST NOT contain another version header,
project wrapper, or project description. Includes MUST occur at document level,
never inside an element body or typed field value. For example::

    arcadia 4;
    project rescue "Rescue" id "00000000-0000-4000-8000-000000000001" {
        description "Operational model";
        include "oa/participants.arcadia";
        include "oa/activities/activity_respond.arcadia";
        include "views/diagram_operational.arcadia";
    }

An include path is relative to its containing file. ``.`` and ``..`` segments
normalize within the virtual project root; escaping that root is an error. Bundle
keys MUST already be normalized, use forward slashes and the lowercase
``.arcadia`` suffix, and contain no empty path segments. Absolute, drive, UNC,
URL, backslash, control-character and non-portable Windows device paths are
rejected. Paths ending a segment in dot or space are rejected. Keys that collide
after Unicode lowercasing are rejected. References use the exact normalized
spelling; filesystems do not participate in resolution.

Every reachable file is compiled once, including through repeated or diamond
includes. Include cycles are errors with their chain and including location.
Missing files, duplicate JSON file keys and supplied unreachable files are
errors. The compiler MUST NOT read local files, network resources, environment
variables or the process working directory. The native loader is responsible
for translating a directory into this explicit bundle and enforcing physical
path ownership; the pure compiler grants no filesystem access.

``inspect_includes(path, source, is_entry, limits)`` supports native discovery
before all included documents are available. It uses the same strict entry or
fragment syntax parser, applies document byte/record and local include-edge
budgets, and returns normalized targets with exact directive spans. Comments and
strings that mention include syntax are not directives. A syntax failure anywhere
in the document returns located errors and no partial include list; it does not
scan arbitrary token strings after failure. Global references and engineering
validation are deferred to bundle compilation. A native loader MUST retain the
unparsed draft on failure and enforce aggregate bytes/files/depth/edges while
loading; this inspection call does not own or inspect a directory.

All files share the existing global alias/UUID namespace. Files add no semantic
namespace, implicit parent, allocation or port contract. Forward references work
across files. Duplicate aliases and identities report the offending declaration
and the previous declaration location, including collisions with the project.
Moving a declaration between files MUST preserve the resulting model and UUIDs.

``ProjectLimits`` adds file, include-depth and include-edge budgets to the existing
``language::ParseLimits``. Model bytes, graph objects, records and ordered
references are counted across the complete bundle, never reset per file. The
default candidate policy is 256 files, depth 32 (entry depth zero), 1,024 include
edges and 64 MiB combined UTF-8 source. Other model counts remain caller policy.
Repeated include edges count toward the edge budget; file contents count once.
The compiler admits input and stabilized output against the same limits.

``compile_bundle`` returns either located errors and no candidate, or a
``CompiledSourceProject`` with the validated ``DecodeResult``, stabilized bundle,
derived ``SourceIndex`` and located engineering warnings. It inserts explicit
UUID clauses into headers that omitted them without otherwise reformatting the
input. The returned source index and diagnostics address the returned stabilized
bytes. Clients MUST retain that bundle after successful Apply; recompiling the
original buffers with missing identities would generate fresh identities.

Bundle diagnostics wrap the existing Diagnostic with a logical path and optional
related locations. Declaration spans, own-field value spans, lexical owners,
declaration bodies, project header/body/description spans and resolved includes are available in
the source index. Columns remain one-based Unicode scalars within the real file;
spans never refer to concatenated or synthetic source.

Single-string parse/lint APIs remain filesystem-free and reject include
statements with an explicit source-bundle requirement. V1/V2 single-file imports
continue through explicit migration. Modular projects accept source3 and source4;
source3 is upgraded as one project, preserving file paths, includes, nested
ownership and comments through targeted token edits. Generated allocation
declarations stay in their interface's existing file. Individual include fragments
are not independently migrated. Missing UUIDs stabilize before migration so new
allocation identities are deterministic on subsequent compilation.

6.2. Nested element ownership
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

An element body MAY contain element declarations interleaved with its ordinary
fields. Its immediate enclosing element is its lexical owner. For example::

    sa_actor harbor "Harbor operator" {
        sa_port radio "Radio" {
            contract { Component { kind Standard; orientation Unset; }; };
        }
    }

Nesting expresses semantic ``Element.parent`` only. It does not allocate a
function to an actor or System, deploy equipment, or add or reorder occurrence
references in an owner's explicit ``steps`` list. Those relationships retain
their explicit records and fields. The
same parent-kind, layer and acyclicity constraints apply as for flat source.
Only element declarations can nest: relations, diagrams, placements, artifacts,
traces and include directives remain at document level. A typed record value
contains fields, never element declarations.

If a nested declaration omits ``parent``, it inherits its lexical owner's UUID.
An explicit ``parent`` MUST reference that owner; another reference or ``none``
is an error located at the parent value. At document level, omitted ``parent``
means ``none`` and explicit parent references retain their existing meaning.
Nesting creates no alias namespace. Aliases and UUIDs remain globally unique,
and references can target nested declarations before or after their definition
or from another included file. Identity stabilization inserts missing UUIDs in
nested headers just as it does in document-level headers.

Source positions distinguish each declaration's own fields from its descendants'
fields. A declaration's body span encloses its lexical children as well as its
own fields. An inherited parent has no synthetic source token or field span.
Located diagnostics and editor navigation MUST address real source bytes.

7. Canonical output and source tooling
--------------------------------------

For every structurally valid Project P, parsing its canonical output MUST
reproduce P, and canonicalizing that result MUST be idempotent:

.. code-block:: text

    parse(format(P)) == P
    format(parse(format(P))) == format(P)

Equality includes all persistent fields, stored aliases, body variants,
references, placements, artifacts, trace baselines, and ordered occurrences.
Map order is irrelevant. The writer preserves signed coordinate zero and
emits decimals that recover the original binary32 value.

Canonical output uses LF, four-space indentation, explicit identities,
defaults, and JSON-escaped strings. Elements are emitted in UUID order, then
relations; diagrams and their nodes follow, then artifacts, then traces.
Fields use deterministic name order. Output ends with LF. Alias values come
from the model, never from regenerating display names.

Comments and input whitespace are lexical trivia to the canonical writer.
Canonical export does not preserve them. The source-project editor and compiler
preserve authored comments and whitespace on Apply while inserting missing
identities. Explicit canonical export remains a separate operation. Project
update/export distinctions are described below.
Camera position, selection, source cursor, and undo history are not stored
model fields. Layout, collapse state, and manual port anchors inside
NodePlacement are model data.

The pure ``format_bundle`` exporter uses the same typed declaration writer as
``format``. It creates a small entry file, layer/category modules, separate
activity/function roots and behavior contexts, and separate diagrams with their
placements. Ports follow their owner; relations follow their source/context.
Module names derive from persistent aliases, not display names or UUID chunks.
Declarations are ordered by semantic ancestry and alias, with owners before
descendants. Case-colliding generated file names receive deterministic numeric
suffixes. When more than 240 distinct modules would be produced, aggregate
folders consolidate into category modules to avoid thousands of tiny files.
For every valid P, ``compile_bundle(format_bundle(P)).decoded.project == P``
when the caller supplies sufficient admission budgets.

``update_bundle(compiled, P, limits)`` is a core source update facility. It
checks that the compiled document/index/model snapshot still agrees, preserves
untouched document bytes, and recompiles the result before returning it. Flat
declarations retain their document-level position and file when reparented.
Nested declarations move into their new semantic owner's body, across files if
necessary; becoming a root moves them to document level. New children prefer
their owner's body. New roots use canonical module rules and add an explicit
entry include.
Removed declarations do not delete source files. Alias changes also update
declarations containing references to that alias. Invalid or stale updates
return errors without mutating the input workspace.

A moved declaration carries its contiguous leading comment block and comments
following its closing brace on the same line. A blank line separates a comment
block from the following declaration; such gap comments stay in the enclosing
scope. Comments trailing a preceding field likewise stay with that field.
Deletion retains annotations in the nearest surviving original scope and
separates them from adjacent declarations with blank lines. Surviving descendants
move with an enclosing declaration unless their own semantic parent changes;
independent parent and child moves are resolved together. Leading whitespace
adjusts to the destination scope; other token spelling remains authored.

Updates MUST NOT canonicalize surviving declarations in full. Header, field and
nested value positions are derived from the existing syntax parser. Unchanged
fields retain their exact bytes, including field order, omitted defaults, number
and string spelling, comments, indentation and line endings. Changed literals and
references replace only the required tokens; a reference with comments between
``@`` and its alias retains those comments. A previously omitted default is
inserted only if its semantic value changes. New fields use the existing record's
indentation and line-ending style without reformatting its other fields.

A constructor tag change retains shared nested fields in place. Removed fields,
constructors and declarations remove syntax tokens, retaining their comments
and whitespace as annotations in the enclosing record or file. A complete value
shape replacement likewise retains the old value's comments beside the new value.
The updater does not invent an engineering interpretation for orphaned comments.

List entries use stable referenced UUIDs, message-value ``field`` references or
lifeline-binding ``referenced`` identities when these keys are unique. Literal
entries use their literal value. Retained entries carry their authored value
text and inter-separator comments when reordered. Trivia after a preceding comma
belongs to the following entry; trivia before the next comma belongs to the
preceding entry. Separators are reconstructed before trailing line comments so
that the result remains valid syntax. Unkeyed or duplicate-key lists use source
positions. Removed-entry comments remain as annotations in the list; newly added
entries receive formatted syntax. These rules preserve comments without silently
attaching new semantic identity to an ambiguous list entry.

Retained declaration UUIDs remain unchanged unless explicitly edited. Source
ownership remains unchanged except for the nested relocation rules above.
Records match by UUID, not display name or alias. The model-only ``update_bundle`` treats an
explicit UUID replacement as removal and creation. The desktop instead uses
``update_bundle_with_identity_map`` with the surviving-record provenance from
``prepare_command``: explicit identity edits and sequential identity swaps MUST
preserve each original declaration's owning file and attached source trivia,
unless the same edit changes its nested semantic parent and requires relocation.
New records have no original identity mapping. The mapping MUST be injective,
refer to existing records and preserve record classes. The
result is recompiled, reindexed and checked for equality with the requested
model before return. Equal model values do not imply a no-op if command
provenance removes or replaces a declaration; the resulting model and source
bundle together determine whether the accepted state changed.
Source changes needed for undo are workspace history data;
callers should retain complete compiled workspace snapshots rather than infer
past omission/layout choices from a model-only undo.

Canonical modular export is a distinct, explicit operation and does not promise
original comment/layout preservation. The native editor integrates per-file
drafts and source-project saves as specified below. The pure compiler performs
no filesystem access. REST ``source-project`` endpoints exchange the accepted
bundle with all its files. One revision and ETag protect the model and source
together, including source-only comment edits; stored envelopes contain both.
Canonical ``source`` endpoints remain explicit model interchange. The precise
transport, admission and persistence contract is in ``api.rst``.

``parse`` returns a complete structurally validated candidate or error
diagnostics and no model. Engineering warnings alone MUST NOT reject a valid
candidate. Parsing may stop at the first syntax/conversion error. Diagnostics
contain severity (Error or Warning), a code identifying the diagnostic category,
optional object UUID and field path, a message, one-based line and Unicode-scalar
column, and a half-open UTF-8 byte Span. Tabs count as one scalar; LF advances
the line. Structural errors and engineering warnings point to a relevant
declaration or field when available. Clients MUST use severity/code rather than
parse the human message; diagnostic wording is not a stable machine API.

``parse_limited`` and ``decode_limited`` accept caller admission budgets for
input bytes, graph objects, total persistent records and ordered step references.
Counts MUST be checked before expensive structural validation; migration budgets
include generated records before migration. Exceeding a budget does not mutate
the model. Admission limits are host capacity policy, not hidden restrictions
on otherwise valid language semantics; ordinary ``parse`` has no such model
size policy. The REST service's fixed policy is specified in ``api.rst``.

``tokenize`` is tolerant: it covers every input byte with nonoverlapping
TokenSpans, including incomplete strings and invalid characters. Token classes
are Keyword, Identifier, String, Number, Comment, Punctuation, Whitespace,
and Error. ``@`` is punctuation followed by the referenced alias token.
``lint`` returns syntax, structural and engineering warning diagnostics for the
current draft without adopting a model. A valid unallocated SA function, for
example, can report missing System and allocation warnings before Apply.
``analyze``
combines tokens, diagnostics, and declaration spans with alias, optional
explicit UUID, and declaration kind. A malformed later declaration does not
erase earlier declaration spans. None of these APIs mutates a session.

8. Graphical parity and transactional editing
---------------------------------------------

The graphical editor MUST expose all persistent properties through typed
controls, including optional references, lists, variant constructors, aliases,
IDs, nested parent changes, diagram placements, artifact metadata, and trace
baselines. Inspector controls may provide operations unavailable as a canvas
gesture. Exact strings, including tabs and line breaks in names, must remain
editable through an appropriate escaped-string input when ordinary widgets
cannot represent them.

A move into a functional parent, an allocation onto a performer, and a diagram
placement move are different transactions. The UI MUST identify which operation
will occur. Reparenting work must not silently change its performer. A computed
parent summary must remain distinguishable from direct allocation. A collapse
operation hides presentation detail without deleting the children.

Applying a reachable source bundle is one model transaction. Graphical edits
update its owning declarations through the source-preserving updater. Both
paths validate their candidate and model/source equality before
adoption; failure preserves the committed model, save baseline, and history.
Incomplete but structurally safe references and coverage gaps produce warnings
that allow continued authoring and saving.

Source and property drafts contribute to unsaved-work state. Invalid drafts
MUST NOT be replaced by generated text or lost on ordinary navigation. Undoing
source-buffer edits and undoing committed model changes are distinct operations.
Successful source Apply creates one model-history entry. Undo/redo restores
the full stored model and exact accepted source bundle, including aliases,
trace links, occurrence identity, comments and file ownership. Each open file
retains its own unapplied draft across file navigation. Located lint diagnostics
refer to the raw draft bytes rather than an identity-stabilized intermediate.
Property editing MUST NOT silently discard a pending source draft.

Changing a graphical UUID must atomically update map keys and every reference.
Alias changes preserve UUID identity. Deletion must account for owned elements,
incident relationships, ordered occurrences, and diagram placements without
leaving dangling references. Deleting a sequence does not delete the semantic
exchanges it references. Cross-layer counterparts are not owned descendants.
Text edits describe a complete candidate and must remove or repair dependencies
explicitly before Apply succeeds.

9. Persistence and explicit migration
-------------------------------------

Loading detects JSON from its first non-whitespace ``{``; other input is parsed
as Arcadia source regardless of filename. Saving an ``.arcadia`` suffix uses
the accepted V4 source bundle (``arcadia 4``); other suffixes use V4 JSON. Both forms preserve the same
complete model. JSON rejects unknown fields throughout typed payloads,
duplicate maps, unsupported versions, and structural errors.

The schema-3 reader freezes the changed data payloads and old constructor catalog;
schema-4 fields/constructors MUST NOT be silently accepted as schema 3. Existing
typed PortContract records remain unchanged. Information/Material/Energy retain
their nature with Unspecified mechanism; old Event becomes Information with Event
mechanism. Old bounds/defaults/supertypes are absent, String length is 0..unbounded,
and fields retain type/multiplicity/unit with Unspecified direction, Value storage
and no explicit default. No physical mechanism or direction is guessed.

Each old Interface.items entry generates one InterfaceItemAllocation with its
original zero-based order and Unspecified protocols. UUID v5 uses the project UUID
namespace and name ``v2-interface-item:<interface-uuid>:<item-uuid>`` consistently
for schema-2 and schema-3 inputs. Aliases are deterministic and collision-free;
collision with any existing identity is an error rather than an overwrite. Existing
interface/item identities remain intact. Old nested message values acquire a path
only when exactly one valid path exists. Ambiguous paths, including recursive
reuse, retain the old annotation with a located warning rather than guessing.

Migration reports additionally contain ``semantic_defaults`` (object, field,
description), ``generated_interface_allocations`` (id, source, target, alias,
order) and ``unresolved_message_values`` (occurrence, field, reason). These new
report arrays default empty when reading older responses. Version 1 normalizes
through V2, then the typed-port V3 adapter and this shared V4 data upgrade. Original
V2 and real V3 fixtures remain unchanged compatibility inputs. Current exports
always use version 4; reading/migrating alone never saves or changes revisions.

Native source Save All checks the original contents of observed dependencies
before staging writes. Existing target files are replaced individually and the
entry is installed last. A transaction journal records originals and intended
writes; explicit recovery restores originals only when current bytes match a
known transaction state. Unrelated external contents MUST NOT be overwritten
by recovery. Missing entry files and lint updates MUST NOT hide a detected
pending-recovery action. This is not cross-process compare-and-swap or atomic
multi-file visibility. Save failures retain the accepted model and drafts.
Detached dirty buffers MUST be reattached or reverted before source saving;
JSON export does not replace the source project's saved baseline or location.

The frozen schema-2 reader retains staged compatibility defaults. Old Scenario bodies without a
``viewpoint`` retain MixedLegacy, and old Exchange occurrence bodies without
``message_kind``, ``reply_to`` or ``values`` retain Asynchronous, none and [].
Old StateTransition bodies without ``transition_kind``, ``triggers`` or
``effects`` retain External, [] and []; existing ``trigger``, ``guard`` and
``effect`` strings are preserved exactly. Source declaration defaults and JSON
compatibility defaults MUST agree; no event or work reference is inferred from
legacy prose.
This compatibility applies equally to source and JSON. It adds explicit
defaults without changing any prior identity, name, reference, order, geometry,
artifact or trace field. The original V2 golden source/JSON in
``crates/oa-core/tests/fixtures`` MUST NOT be regenerated from the newer demo;
they verify that every old persistent field survives. Canonical output writes
the added defaults through the explicit schema-3 and schema-4 migrations.

The V2 adapter preserves every original identity, alias, prose, parent,
placement, trace and artifact. Generic ports become Legacy with their original
direction/interface record. Existing FE endpoint IDs remain unchanged. For
only a missing endpoint it creates an item-bearing FunctionOutput/Input owned
by the already specified function, using UUID v5 in the project namespace
with ``v2-fe-port:<fe-id>:source`` or ``v2-fe-port:<fe-id>:target``. Every
identity category is checked for collision; collisions are located errors,
never overwrites. New aliases follow the deterministic collision-free alias
convention. Every old boundary exchange receives ``oriented true``; no
PortAllocation is inferred.

``decode_with_report``, ``load_with_report`` and their limited/source variants
return the model plus a migration report containing original unresolved port
IDs, required resolutions, generated endpoint IDs/aliases/owners, and boundary
orientation defaults. REST import, replacement, candidate validation and
project snapshots surface the same report. Opening an old server envelope
preserves its resource ID/revision and bytes; the next explicit edit writes
schema 4. Model/source exports contain only the canonical model. Migration
reports are transient response metadata, not persisted model facts.

Admission checks account for generated objects and their references before
construction. Both the old V2 contract and the resulting V4 graph are
validated. Migration itself does not save or increase a stored revision.
Canonical second saves are stable. V1 normalization produces the frozen V2
model and then uses these same V2-to-3-to-4 stages.

Save validates a snapshot, writes a sibling temporary file, flushes it, and
atomically replaces the destination. Failed serialization or validation must
not truncate a previous file. External concurrent-writer conflict resolution
and crash recovery are outside this storage contract.

Version 1 text and JSON are accepted through an explicit adapter. Migration
preserves project, element, and relation UUIDs; names and descriptions; relation
endpoints; all coordinates; and exact process exchange order and repetition.
Allocation and Interaction remain their corresponding V2 relations.
Contribution becomes CapabilityInvolvement; the old Process-to-Capability
Realization becomes ChainInvolvement, retaining its ID and name.

Legacy positions become nodes in one OA Overview diagram. Each old process
step becomes a distinct occurrence element owned by its process. Generated
diagram, node, and occurrence identities use UUID v5 with the old Project ID
as namespace and deterministic role/index names. If such an identity collides,
a deterministic suffix is tried. Reimporting the same fully identified V1
input therefore yields the same V4 result, including repeated occurrences.
No System or SA content is invented during an OA-only migration.

V1 source aliases are retained when valid in V2. JSON imports generate the old
canonical aliases deterministically. Any alias newly reserved in V2 is changed
by a deterministic suffix; aliases were not persistent V1 semantic data.
A project alias and aliases for new records are generated without colliding
with retained aliases. The existing bundled V1 text and JSON migrate to equal
V4 projects. Future versions remain errors until a separate migration exists.

10. Complete authoring example
------------------------------

This document intentionally omits IDs for new authoring. Apply and Save write
the assigned UUIDs explicitly.

.. code-block:: text

    arcadia 4;
    project harbor "Harbor assistance" {
        oa_entity control "Harbor control" {}
        oa_activity assess "Assess assistance" {}
        allocation performs "Performs" {
            source @control;
            target @assess;
        }
        sa_system service "Harbor assistance system" {}
        sa_function evaluate "Evaluate assistance request" {}
        allocation system_performs "Performs" {
            source @service;
            target @evaluate;
        }
        trace assessment_trace "Operational realization" {
            oa @assess;
            sa @evaluate;
            kind Refinement;
        }
        diagram operational_view "Operational allocation" {
            layer Oa;
            projection Allocation;
        }
        node assessment_card {
            diagram @operational_view;
            element @assess;
            position { x 360; y 70; };
        }
    }

11. Verification and primary references
---------------------------------------

``cargo test -p oa-core --test language --locked`` verifies typed round-trips,
stable aliases, strict syntax/reference failures, Unicode source spans,
deterministic V1 migration, occurrence identity, coordinate bits, persistence,
and model-history integration. Core validation and session tests cover domain
mutation rules; native UI and API verification are separate evidence.

The research record in ``docs/research-v2-semantics.md`` distinguishes method
requirements from Studio design choices and links to original UI evidence.
The following sources inform this language without extending its scope:

* `Arcadia Q&A <https://mbse-capella.org/arcadia-qna.html>`_
* `Arcadia language reference <https://mbse-capella.org/resources/arcadia-reference/Arcadia%20Language%20-%20MetaModel.pdf>`_
* `Capella allocation guide <https://github.com/eclipse-capella/capella/blob/master/doc/plugins/org.polarsys.capella.ui.doc/html/05.%20Diagram%20Management/Allocating%20Functions%20on%20Components.mediawiki>`_
* `Capella iterative transitions <https://github.com/eclipse-capella/capella/blob/master/doc/plugins/org.polarsys.capella.transitions.doc/html/11.%20Iterative%20Transitions/11.1.%20Concepts.mediawiki>`_
* `W3C XML 1.0 characters <https://www.w3.org/TR/xml/#charsets>`_