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The Structural Index provides agents with global awareness of the score’s structure without requiring the full content. See: /MRS-Specification-RFC#9-structural-index

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Purpose

Agents operating on a focused region need to understand:
  • Where they are in the overall form
  • Key areas and tempo changes
  • What instruments are active
  • Rehearsal mark locations
The Structural Index provides this without loading the entire score.

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Index Structure

(structural-index
  :hash "sha256:abc123..."
  :source-hash "sha256:def456..."
  
  :form
    (sections
      (section :type exposition :measures #uuid "..." #uuid "..." :display [1 64])
      (section :type development :measures #uuid "..." #uuid "..." :display [65 120])
      (section :type recapitulation :measures #uuid "..." #uuid "..." :display [121 180]))
  
  :key-areas
    ((region :key C :mode minor :measures #uuid "..." #uuid "...")
     (region :key Eb :mode major :measures #uuid "..." #uuid "...")
     (region :key G :mode minor :measures #uuid "..." #uuid "..."))
  
  :tempo-map
    ((tempo :bpm 120 :measures #uuid "..." #uuid "...")
     (tempo :bpm 80 :text "Adagio" :measures #uuid "..." #uuid "..."))
  
  :players-summary
    ((player woodwind-2 :instruments [flute-2 piccolo] :active-measures [...])
     (player clarinet-1 :instruments [clarinet-bb] :active-measures [...]))
  
  :rehearsal-marks
    ((mark "A" :measure #uuid "..." :number 16)
     (mark "B" :measure #uuid "..." :number 32)
     (mark "C" :measure #uuid "..." :number 48))
  
  :total-measures 180
  :total-duration "12:34")

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Index Fields

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Form

:form
  (sections
    (section :type exposition :measures #uuid "..." #uuid "..." :display [1 64])
    (section :type development :measures #uuid "..." #uuid "..." :display [65 120]))
FieldDescription
:typeSection type (exposition, development, coda, etc.)
:measuresUUID range
:displayHuman-readable measure numbers

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Key Areas

:key-areas
  ((region :key C :mode minor :measures #uuid "..." #uuid "...")
   (region :key Eb :mode major :measures #uuid "..." #uuid "..."))

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Tempo Map

:tempo-map
  ((tempo :bpm 120 :measures #uuid "..." #uuid "...")
   (tempo :bpm 80 :text "Adagio" :measures #uuid "..." #uuid "..."))

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Players Summary

:players-summary
  ((player woodwind-2 :instruments [flute-2 piccolo] :active-measures [...])
   (player clarinet-1 :instruments [clarinet-bb] :active-measures [...]))

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Rehearsal Marks

:rehearsal-marks
  ((mark "A" :measure #uuid "..." :number 16)
   (mark "B" :measure #uuid "..." :number 32))

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Index Derivation

The structural index can be:
SourceDescription
DerivedAutomatically computed from the score
EnhancedEnriched by analytical overlays
AnnotatedManually labeled (form, section names)

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Automatically Derived

  • Measure count
  • Key signature changes
  • Tempo changes
  • Rehearsal marks
  • Player/instrument activity

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Requires Analysis/Annotation

  • Form labels (exposition, development, etc.)
  • Thematic regions
  • Phrase structure

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Usage in Working Sets

Working Set Envelopes include a reference to the structural index: 
(working-set
  ...
  :structural-index-ref "sha256:abc123..."
  ...)
Agents can use this to:
  • Understand where they are in the form
  • Check what other instruments are active
  • Find rehearsal marks for reference
  • Understand key and tempo context

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Index vs Context Views

Structural IndexContext Views
Global overviewTask-specific detail
Read-only summaryMay include content
Always availableSelected per task
Score-level infoRegion-level info
Both are used together: the index for global awareness, context views for task-specific information.