Issue 1830 in lilypond: \change only works with certain contexts

[lilypond]

Status: Accepted
Owner: address@hidden
Labels: Type-Defect

New issue 1830 by address@hidden: \change only works with certain contexts
http://code.google.com/p/lilypond/issues/detail?id=1830

After realizing that a VerticalAxisGroup is created at the onset of certain  
contexts, I asked myself: "what if these contexts were moved to another  
context?"  The code below does just that (in theory) and does not complain,  
except that there is no change of PianoStaff.

<<
\new PianoStaff = "foo" <<
\new Staff { a' b' c' d' }
\new Staff { a' b' c' d' }

\new PianoStaff = "bar" <<
\new Staff { a' b' c' d' }
\new Staff { a' b' \change PianoStaff = "foo" c' d' }



It seems that some contexts should be marked as change-friendly and others  
as not, and those that aren't should issue a warning when the  
change-iterator comes a-knockin' on their door.

Cheers,
MS