music function refactoring

[Erik Sandberg]

Hi,

I'm performing some refactorings, which aim to fix the "\partcombine cannot be 
inside \relative" bug mentioned in section 8.3.1 (plus other similar 
problems).

My plan is:
1. Postpone the applying of music functions: Store all music functions as 
music wrappers, and apply the functions after parsing, using a new 
toplevel-music-function called apply-music-functions!. This function 
recursively searches for music functions to apply, and when it finds one, it 
recurses into its arguments before applying it.

2. Make \relative a music function, but prohibit apply-music-functions! from 
recursing into its argument.

3. Practical utility functions that work well in \relative mode can be 
written, for example:
\relative { c d \interpolate { c c' } b c }
=> { c' d' {c' d' e' f' g' a' b' c''} b' c'' }

AFAICS (1) has minor negative consequences: There will be problems with 
relying on side-effects of music functions. For example, variables defined by 
music functions can't be used until the next top-level music expression 
(still possible with inline #(..)-functions), and it causes problems if you 
want to do side-effects on the parser (it will be confusing to mix #(foo ...) 
functions with \foo functions, since all \foo functions are called after all 
#(foo ...) functions).

-- 
Erik