Issue 1902 in lilypond: scheme engravers cause "warning: Attempting to remove nonexisting listener."

[lilypond]

Status: Accepted
Owner: ----
Labels: Type-Defect

New issue 1902 by address@hidden: scheme engravers  
cause "warning: Attempting to remove nonexisting listener."
http://code.google.com/p/lilypond/issues/detail?id=1902

Reported by Reinhold:

"If one uses a scheme engraver that instantiates itself for each context,  
i.e.
   \consists #(lambda (context) ...)
then using listeners will cause the warning:
   "Warnung: Attempting to remove nonexisting listener.""

\layout {
  \context {
    \Voice
    \consists
      #(lambda (context) `((listeners . ((note-event . ,(lambda (eng ev)  
#t))))))
    % This works, because it does not create a copy for each voice:
%     \consists
%       #'((listeners . ((note-event . ,(lambda (eng ev) #t)))))
  }
}

\relative c'' { <<
  { c4 } \\ { c4 }

}