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MRS uses a unified semantic model expressed through two formats: MRS-S (storage) and MRS-Ops (mutation). See: /MRS-Specification-RFC#11-data-model

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Core Type Hierarchy

Score
├── meta: Metadata
├── players: List<Player>
├── instruments: List<Instrument>
├── measures: List<Measure> | movements: List<Movement>
├── spans: List<Span>
├── overlays: List<Overlay>?
├── structural-index: StructuralIndex?
└── alternatives: List<Alternative>?

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Player-Instrument-Staff Model

MRS uses a three-level model for professional orchestration semantics.

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Player

Player
├── id: Identifier
├── name: String
├── instruments: List<InstrumentId>
└── default: InstrumentId
A Player represents a performer who may play multiple instruments (doubling). 
(player woodwind-2
  :name "Flute 2 / Piccolo"
  :instruments [flute-2 piccolo]
  :default flute-2)

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Instrument

Instrument
├── id: Identifier
├── name: String
├── abbr: String
├── family: InstrumentFamily
├── staves: List<Clef>
├── transposition: Interval | none
├── range: [Pitch, Pitch]?
├── kit: List<KitItem>?  ; percussion only
└── staff-connect: ConnectType?
An Instrument is a specific sound source with fixed transposition and range. 
(instrument clarinet-bb
  :name "Clarinet in Bb"
  :abbr "Cl."
  :family woodwinds
  :staves [treble]
  :transposition (down M2)
  :range [E3 C7])

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Instrument Families

woodwinds brass strings percussion keyboards voices choir other

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Measure

Measure
├── id: UUIDv7              ; Stable identity
├── number: Integer         ; Display number
├── beat-start: Rational    ; Orchestrator-computed
├── time: TimeSignature?    ; Inherits if omitted
├── key: PitchClass?        ; Inherits if omitted
├── mode: Mode?             ; Inherits if omitted
├── tempo: Integer?         ; Inherits if omitted
├── tempo-text: String?
├── rehearsal: String?
├── barline-left: BarlineType?
├── barline-right: BarlineType?
├── directions: List<Direction>
└── content: Map<InstrumentId, InstrumentContent>

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Measure Identity

  • :id — Stable UUIDv7, survives structural changes
  • :number — Display only, can change on insert/delete
  • :beat-start — Orchestrator-computed, absolute position

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Attribute Inheritance

AttributeInherits?
:timeYes, until changed
:key, :modeYes, until changed
:tempo, :tempo-textYes, until changed
:rehearsalNo (single-measure)
:barline-*No (per-measure)

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Event

Event
├── beat: Rational          ; Position within measure (0-indexed)
├── pitch: PitchExpr        ; Pitch, chord, or rest
├── id: UUIDv7              ; Stable identity
├── dyn: Dynamic?
├── art: Articulation?
├── orn: Ornament?
├── tech: Technique?
├── lyrics: List<LyricSyllable>?
├── grace: Boolean?
├── cue: Boolean?
├── cue-source: InstrumentId?
└── ...other properties

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Event Position

Events have structural position (beat within measure). The orchestrator computes absolute position: 
event.:at = measure.:beat-start + event.beat
In MRS-S storage, :at is optional (can be computed). In MRS-Ops, agents provide :beat; orchestrator computes :at.

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Pitch Expression

TypeExampleDescription
Single noteC4.qC4 quarter note
Chord[C4 E4 G4].qC major triad, quarter
Restr.qQuarter rest

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Span

Span
├── id: UUIDv7
├── type: SpanType
├── from: UUIDv7?           ; Start event (for from/to spans)
├── to: UUIDv7?             ; End event (for from/to spans)
├── events: List<UUIDv7>?   ; Event list (for beams)
├── boundary-entry: Boolean?
├── boundary-exit: Boolean?
└── ...type-specific attributes

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Span Types

TypeEndpoint StyleDescription
slurfrom/toPhrasing slur
tiefrom/toTie between same pitches
hairpinfrom/toCrescendo/diminuendo
beamevents listBeam grouping
ottavafrom/toOctave transposition
pedalfrom/toPiano pedal

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Span Immutability

Once created, span endpoints (:from, :to, :events) are immutable. To change endpoints, delete the span and create a new one.

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Identifier Rules

TypeFormatExample
UUIDUUIDv7#uuid "018c3f40-0001-..."
Instrument ID[a-z][a-z0-9-]*clarinet-bb, flute-1
Player ID[a-z][a-z0-9-]*woodwind-2
Voice IDv1-v4 or :rh/:lhv1, :rh
Tmp-IDString starting with letter"e1", "span-a"

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Rational Numbers

All temporal positions use exact rationals: 
Canonical forms:
  0, 1, 2, ...       ; Integers
  2+1/2              ; Two and a half
  0+1/3              ; One third
  3+3/4              ; Three and three quarters

NOT permitted:
  2.5                ; Decimal
  0.333              ; Decimal
Implementations MUST treat rationals as exact; no floating-point approximation.

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Voices and Layers

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Voices

Independent melodic/rhythmic streams within an instrument:
  • v1 through v4 for single-staff instruments
  • :rh/:lh for keyboard instruments (then v1, v2 within each)

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Layers

Same-rhythm sub-structure within a voice: 
(v1
  (layer 1
    (: 0 C4.q :id #uuid "...") (: 1 D4.q :id #uuid "..."))
  (layer 2
    (: 0 E4.q :id #uuid "...") (: 1 F4.q :id #uuid "...")))

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HarmonyPlan Layer

The HarmonyPlan is an embedded optional layer for chord symbols and harmonic structure: 
(harmony-plan
  (region :measures #uuid "..." #uuid "..."
    (chord :beat 0 :symbol Cm :function i)
    (chord :beat 2 :symbol Fm :function iv)))
This enables:
  • Composing harmony before notes
  • Validating notes against harmony
  • Extracting harmonic context for agents