% Thanks to: Dan Eble <address@hidden>
% Usage:
%
% aNotes = \relative c' { c1 e1 g1 }
% bNotes = \relative c' { }
%
% \ifThenElse \bNotes \aNotes \bNotes
%
% Note the test is on emptyness, not definedness.

#(define-public (music-empty? m)
   (let ((void-music (ly:music-property m 'void)))
     (or (and (not (null? void-music)) void-music)
	 (and (null? (ly:music-property m 'element))
	      (null? (ly:music-property m 'elements))))))

ifThenElse =
#(define-music-function
   (parser location if-music then-music else-music)
   (ly:music? ly:music? ly:music?)

   (if (music-empty? if-music)
       else-music
       then-music))