Getting the height of a system.

[Joe Neeman]

We discussed this a bit a long time ago but I'm getting close to actually 
implementing something so I thought I'd bring it up again.

Getting the Y-extent of a system is potentially destructive, causing all sorts 
of non-undoable caching and possibly suiciding of grobs. So I need to do make 
a copy of the system before I can get its Y-extent.

The problem is that there are a large number of possible lines in a score. To 
copy a system, I need to do a full grob substitution for every grob in that 
system. If each bar could be in up to N possible lines, I need to do N full 
copies of the entire score and N full grob substitutions in the 
object_alist_. If we can fit up to K bars to a line then N is approximately 
K(K-1)/2. In the scores I typically work with, N would be around 50-60 so 
this is getting pretty expensive.

So I propose the following instead. I don't know a great deal (yet) about the 
C++ internals of grobs so I'd appreciate knowing if this is unworkable before 
I spend lots of time trying to implement it.

1) add a virtual Grob *save () and virtual void restore (Grob*) function to 
every C++ Grob subclass. save() would be a bit like clone except that it 
copies the object_alist_ and, for example, broken_intos_ in the case of a 
Spanner.
Grob *copy = me->save ();
// make some changes to me
me->restore (copy);
and you end up with the same grob you started with.

2) save every grob in the score

3) for each possible system, using the original grobs,
        - work out the Y-extent
        - restore () every grob in that system

The advantage is that we avoid fiddling around with the object_alist_ because 
this clone is never used -- it exists only to save the state of the score 
before we mess it up with calling Y-extent. We also only have to make one 
copy of the system and restore it N times instead of making N copies.

Is this feasible?