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See: /MRS-Specification-RFC#4-11-spans-section and /MRS-Specification-RFC#4-13-directions

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Spans

Spans define relationships between events. All references use UUIDs: 
(spans
  (slur
    :id #uuid "018c3f2c-9abc-def0-1234-567890abcdef"
    :from #uuid "018c3f2a-1234-7890-abcd-ef1234567890"
    :to #uuid "018c3f2b-5678-9012-cdef-0123456789ab"
    :placement above)
  
  (tie
    :id #uuid "018c3f2d-..."
    :from #uuid "018c3f2e-..."
    :to #uuid "018c3f2f-...")
  
  (hairpin
    :id #uuid "018c3f39-..."
    :from #uuid "018c3f3a-..."
    :to #uuid "018c3f3b-..."
    :type crescendo)
  
  (beam
    :id #uuid "018c3f30-..."
    :events [#uuid "..." #uuid "..." #uuid "..." #uuid "..."]))

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Span Types

TypeDescription
slurPhrase marking
tieConnects identical pitches
beamConnects note flags
hairpinDynamic wedge
ottavaOctave transposition
pedalPiano pedal
glissGlissando line
voltaEnding bracket

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Cross-Boundary Spans

Spans crossing envelope boundaries are marked: 
(slur
  :id #uuid "..."
  :from #uuid "..."
  :to #uuid "..."
  :boundary-exit :to #uuid "...")  ; Endpoint outside envelope
Cross-boundary spans are write-protected at their boundary endpoints.

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Directions

(measure :id #uuid "..." :number 45 :beat-start 487+1/4
  
  (dir :type tempo :beat 0 :text "Più mosso" :tempo 132)
  (dir :type dynamic :beat 0 :text "ff" :scope all)
  (dir :type rehearsal :beat 0 :text "B")
  (dir :type segno :beat 0)
  
  (violin-1
    (dir :type text :beat 0 :text "sul G" :placement above)
    ...))

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Multi-Movement Structure

(movements
  (movement 1 :title "Allegro" :key C :mode major :time 4/4
    (measures
      (measure :id #uuid "..." :number 1 ...) ...))
  (movement 2 :title "Adagio" :key F :mode major :time 3/4
    (measures
      (measure :id #uuid "..." :number 1 ...) ...)))