+ ((prall)
+ ; A pralltriller symbol can either mean an inverted mordent
+ ; or a half-shake -- a short, two twiddle trill.
+ ; We implement as a half-shake.
+ (let*
+ ((totallength (ly:music-length music))
+ (newlen (ly:moment-sub totallength (ly:make-moment 3 32)))
+ (newdur (ly:make-duration
+ 0 0
+ (ly:moment-main-numerator newlen)
+ (ly:moment-main-denominator newlen)))
+ (gracedur (ly:make-duration 5 0 1 1))
+ (gracenote (ly:music-deep-copy music))
+ (abovenote (ly:music-deep-copy music))
+ (mainnote (ly:music-deep-copy music))
+ (prall (make-sequential-music (list gracenote abovenote)))
+ )
+ (music-map (lambda (n)
+ (if (eq? 'NoteEvent (ly:music-property n 'name))
+ (set! (ly:music-property n 'duration) gracedur))
+ n)
+ abovenote)
+ (music-map (lambda (n)
+ (if (eq? 'NoteEvent (ly:music-property n 'name))
+ (set! (ly:music-property n 'duration) gracedur))
+ n)
+ gracenote)
+ (music-map (lambda (n)
+ (if (eq? 'NoteEvent (ly:music-property n 'name))
+ (set! (ly:music-property n 'duration) newdur))
+ n)
+ mainnote)
+
+ (map (lambda (y) (ac:up y))
+ (filter
+ (lambda (z) (eq? 'NoteEvent (ly:music-property z 'name)))
+ (ly:music-property abovenote 'elements)))
+ (make-sequential-music (list abovenote gracenote abovenote mainnote))))
+