Trap pure-y-common spanning multiple staves; issue 3359 (issue 13013046)

[janek . lilypond]

the code LGTM; however, could you expand the commit message (or add a
comment in the source) to explain why we want this?  I think that this
comment http://code.google.com/p/lilypond/issues/detail?id=3359#c11 is a
good material and just needs a few rewords.

thanks,
Janek

https://codereview.appspot.com/13013046/