conditional (on-the-fly) music

[Dan Eble]

I’m working on a change that allows this:

  \repeat volta 4 {
    \conditional #(except-in-nth-volta 3) {
       c4^"3rd time tacet” …
    }
  }

It is engraved normally, but when \unfoldRepeats is used for MIDI output, the 
conditional passage is skipped during the 3rd volta.  It does this by checking 
a new context property that is set in unfolded music.

Any feedback?  I am having a hard time finding the right place to put the 
predicate-generator functions "except-in-nth-volta" and "only-in-nth-volta".
— 
Dan