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1.3.3 Doubling a note with slurs (example)

Suppose we want to create a function that translates input like a into { a( a) }. We begin by examining the internal representation of the desired result. 

\displayMusic{ a'( a') }
===>
(make-music
  'SequentialMusic
  'elements
  (list (make-music
          'NoteEvent
          'articulations
          (list (make-music
                  'SlurEvent
                  'span-direction
                  -1))
          'duration
          (ly:make-duration 2 0 1/1)
          'pitch
          (ly:make-pitch 0 5 0))
        (make-music
          'NoteEvent
          'articulations
          (list (make-music
                  'SlurEvent
                  'span-direction
                  1))
          'duration
          (ly:make-duration 2 0 1/1)
          'pitch
          (ly:make-pitch 0 5 0))))

The bad news is that the SlurEvent expressions must be added ‘inside’ the note (in its articulations property).

Now we examine the input. 

\displayMusic a'
===>
(make-music
  'NoteEvent
  'duration
  (ly:make-duration 2 0 1/1)
  'pitch
  (ly:make-pitch 0 5 0))))

So in our function, we need to clone this expression (so that we have two notes to build the sequence), add a SlurEvent to the 'articulations property of each one, and finally make a SequentialMusic with the two NoteEvent elements. For adding to a property, it is useful to know that an unset property is read out as '(), the empty list, so no special checks are required before we put another element at the front of the articulations property. 

doubleSlur = #(define-music-function (note) (ly:music?)
         "Return: { note ( note ) }.
         `note' is supposed to be a single note."
         (let ((note2 (ly:music-deep-copy note)))
           (set! (ly:music-property note 'articulations)
                 (cons (make-music 'SlurEvent 'span-direction -1)
                       (ly:music-property note 'articulations)))
           (set! (ly:music-property note2 'articulations)
                 (cons (make-music 'SlurEvent 'span-direction 1)
                       (ly:music-property note2 'articulations)))
           (make-music 'SequentialMusic 'elements (list note note2))))
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