Optional arguments of a music-function?

[Thomas Morley]

Hi,

I tried to adapt the syntax-trickery David showed with the new
\tweak-definition to an own function.

Trying a code like the tiny example below fails.

proc =
#(define-music-function (parser location name arg)((string?) ly:music?)
  (if name
  #{ \once \override $name #'color = #red  $arg #}
  #{ \parenthesize $arg #}))

\relative c' {
        c1
        \proc "NoteHead" c
        \proc c
}

Obviously I did sth wrong.
Well, I could do:

#(define-public (string-or-music? x)
  (or (string? x) (ly:music? x)))

procII =
#(define-music-function (parser location arg)(string-or-music?)
  (if (string? arg)
  #{ \once \override $arg #'color = #red  #}
  #{ \parenthesize $arg #}))

\relative c' {
        c1
        \procII "NoteHead" c
        \procII c
}

But I'd prefer the first version.
How  to do?

-Harm