Feature request: cross-staff chords

[Pavel Roskin]

Hello!

In my opinion, Lilypond should be able to support chords with  
noteheads on different staves.  This is especially important within  
staff groups for keyboard instruments.

Lilypond can already move a voice from one staff to another.  It can  
connect notes on different staves with beams.  Yet it doesn't allow  
notes from one voice to be on different staves at the same moment.

Lilypond supports cross-staff stems, which provides a workaround and  
thus acknowledges the need to have cross-staff chords.

The problem with the cross-staff stems is that there is no reliable  
way to ensure that they would reach all the noteheads unless the  
distance between staves is fixed.  Fixing the distance between staves  
requires such hacks as negative padding.  Lilypond should be free to  
choose the distance between staves for the best layout and adjust the  
cross-staff stems to look correctly.

It is possible to use overlapping stems to guard against stem  
discontinuity.  However, overlapping parts of stems may look darker  
than non-overlapping parts dependent on the anti-aliasing algorithm  
used to display them.

But it's not just about stems.  Even whole notes could benefit from  
being in a cross-staff chord.  It would make it easier to connect  
those notes to other notes from the same voice.

The most natural syntax would be to allow \change inside chords:

\new PianoStaff <<
  \new Staff = right { <c' \change Staff=left c>4 }
  \new Staff = left { \clef bass s4 }
> >

--
Regards,
Pavel Roskin