divisi staves and notehead merge (issue 3518 revisited)

[Werner LEMBERG]

Please have a look at the `divisi-staves.ly' regression test, which is
the solution to

  https://code.google.com/p/lilypond/issues/detail?id=3518

[Actually, this file contains undocumented features that deserves
documentation due to its importance for orchestral music...]

I want a variation of this, namely solo violin + gli altri:


  boring = \set Staff.keepAliveInterfaces = #'()
  tricky = \unset Staff.keepAliveInterfaces

  violsolo=\relative d' {
    \boring \repeat unfold 100 d4
    \tricky <d g'>2
    \boring \repeat unfold 98 d4
    \bar "|."
  }

  viol=\relative d' {
    \boring \repeat unfold 100 d4
    \tricky d4 d4
    \boring \repeat unfold 98 d4
    \bar "|."
  }

  \score {
    \new StaffGroup \with { \consists "Keep_alive_together_engraver" }
    <<
      \new Staff \with { instrumentName = "Violin solo"
                         shortInstrumentName = "V solo"
                         \override VerticalAxisGroup.remove-empty = ##t
                         \override VerticalAxisGroup.remove-first = ##t
                         \override VerticalAxisGroup.remove-layer = 1
                       } \violsolo

      \new Staff \with { instrumentName = "Violin gli altri"
                         shortInstrumentName = "V"
                         \override VerticalAxisGroup.remove-empty = ##t
                         \override VerticalAxisGroup.remove-first = ##t
                         \override VerticalAxisGroup.remove-layer = 1
                       } \viol
      \new Staff \with { instrumentName = "Violin"
                         shortInstrumentName = "V"
                         \override VerticalAxisGroup.remove-layer = 2
                       } << \violsolo \viol >>
    >>
    \layout {
      short-indent = 2\cm
      indent = 3\cm
    }
  }


Note the missing `\\' in `<< \violsolo \viol >>'.  The attached image
shows the result, which is not yet what I would like to have.  Now my
question: Does lilypond have already a property to make the noteheads
merge even if the stems are in the same direction?  Or can you advise
a better solution?



    Werner