Catch possible non-rhythmic-event music expressions

[Urs Liska]

Hi,

I'm working on a music-function that takes one music argument, 
determines the type of music and dispatches to a styling function that 
will apply either a \tweak, a \once \override or that will wrap the 
music in \temporary \override / \revert expressions.

I got pretty far in the determination of when to use which modification 
type, and I arrived at using the \once \override appraoch for 
non-rhythmic events like key signatures, clefs, time signatures etc., 
which so far seems to work pretty well.

However, now I want to add the functionality to auto-detect the grob to 
override, and I thought checking for the 'types music-property. But 
there arise a few questions:

#(define (infer-item music)
   (let ((types (ly:music-property music 'types)))
     (ly:message "Types: ~a" types)
     (cond
      ((memq 'key-change-event types) '(Staff KeySignature))
      ((memq 'time-signature-music types) '(Staff TimeSignature))
      ((memq 'mark-event types) '(Score RehearsalMark))
      ((memq 'tempo-change-event types) '(Score MetronomeMark))
      ((memq 'music-wrapper-music types) '(Staff Clef)))))

1)
Determining Staff.Clef is obviously not correct since there are other 
music-wrapper-music types around. How can I reliably determine if a 
music expression (which has already been determined not to be sequential 
and not to be rhythmic) consists of a \clef assignment?

2)
How can I know the comprehensive list of type/grob mappings I have to 
support?

3)
What would be a better structure than the growing list of memq 
conditionals, especially if the list should turn out to be significantly 
longer than the five I currently have?

Thanks
Urs