Scheme question: symbol to music

[Johan Vromans]

Stupic question, I assume...

In a scheme function I have a symbol that is the name of a lilypond
expression. How can I get its music value?

E.g.

  ifDefined =
  #(define-music-function
   (parser location sym)
   (symbol?)

   (if (defined? sym)
       ??? return the value of the expression ???
    (make-music 'SequentialMusic 'void #t)))

So I can write:

  \ifDefined #'myNotes

and have it expand to myNotes (if defined), or a void music expression
otherwise.

-- Johan