[bug] \once \override Score.SystemStartBracket holds forever

[Juergen Reuter]

Hi!

Once Score.SystemStartBracket #'transparent has been overridden to ##t, it 
seems it is not possible to revert the effect.  For an example, look at 
section 3.4.2 in the manual (in recent CVS; this is not yet on the web 
site) or, respectively, in the generated file
Documentation/user/out-www/lilypond/lily-2054725167.ly.  As you can see, 
the system start bracket is removed from both lines of score, not just the 
first one.

On a similar vein, in the manual it does not get clear what happens 
with contradicting \override or \set commands (at least I could not find 
something about this issue).  Imagine, for example, two voices that live 
in the same staff.  The first voice sets some context property of the 
staff context or overrides a grob property of a grob that lives in the 
staff context.  The second voice touches the same properties at the same 
score time, but with different values.  Which voice wins?  The manual 
probably should say something about this (or, even better, maybe in 3.1.x, 
lily should issue a warning during interpreting phase about conflicting 
property settings).  Of course, the cleanest solution would be to allow 
property settings only for the current scope of context (which would have 
some major consequences for lily's syntax, though).

On a third vein, I just recognized that \once does not appear in the 
unified index.  It should, I guess (and maybe there should also be a short 
section in the manual that introduces \once).

Greetings,
Jürgen