Regression: music function applied to a bass figure causes syntax error

[Nicolas Sceaux]

Hi,

With lilypond 2.19.3, applying a music function to a bass figure
causes a syntax error.  At least until 2.17.24 (I have not tested
later versions), that was not the case.

Example:

\version "2.19.3"

%% Figured bass
%% change a flat or sharp alteration into natural
%% unless 'ancient-style option is true
naturalFig =
#(define-music-function (parser location fig) (ly:music?)
   (if (eqv? #t (ly:get-option 'ancient-style))
       fig
       (music-map
        (lambda (music)
          (if (eqv? 'BassFigureEvent (ly:music-property music 'name))
              (let ((alteration (ly:music-property music 'alteration)))
                (if (and (number? alteration)
                         (or (= alteration 1/2) (= alteration -1/2)))
                    (set! (ly:music-property music 'alteration) 0))))
          music)
        fig)))

<<
  \new Staff { \clef "bass" c1 }
  %% this used to work, but now causes a syntax error:
  \figures { \naturalFig <_+> }
  %% workaound:
  %\figures { \naturalFig { <_+> } }
>>