Format LilyMusic as string

[Urs Liska]

I have a ly:music? argument and want to store the LilyPond 
representation as a string, that is I want to have a string with the 
content that \displayLilyMusic prints to the console.

From what I know I have to provide a port to which \displayLilyMusic's 
output is redirected and that I can then store. But For some reason 
(well, that includes the "conciseness" of the Guile reference manual 
...) I don't get it done.


\version "2.19.80"
#(define format-ly-music (mus)
  (dont-know-what-to-do-here))

#(define my-var (format-ly-music #{ c' #}))

looks like a MWE for what I need.
From the reference I would have thought that call-with-output-string 
would be what I need, but I don't get it to work as it complains about 
the "unspecified" argument.

Any pointers?
Thanks in advance
Urs