scheme accessors

[Nicolas Sceaux]

I had a cold sweat when running the new 2.1.26, but then read:
"The naming of exported Scheme functions now follows Scheme conventions."
Ouf.

This is just a suggestion:
It may be more idiomatic to define a setter for accessors. For
instance, given the reader ly:music-property and the writer
ly:music-set-property!:

  (define-public ly:music-property
    (make-procedure-with-setter ly:music-property
                                ly:music-set-property!))

you can then use ly:music-property to access the property as before,
but you can also use it with set!:

 ;; return the 'elements property:
 (ly:music-property my-music 'elements)

 ;; set the 'elements property (and return it):
 (set! (ly:music-property my-music 'elements) value)

This is more lispy. Note that you can still use the
ly:music-set-property! procedure (thus it does not involve massive
corrections).

nicolas