Cross-staff beam slope - can it be specified?

[David Sumbler]

\version "2.19.48"

\new PianoStaff {
  <<
    { \new Staff = "upper" {
      \stemDown c'8. \change Staff = lower \stemUp g16
      \change Staff = upper \stemNeutral <c' g>2. } }
    { \new Staff = "lower" {
      \clef "bass" s4  <e c>2. } }
  >>
}


In the above rather silly example, the beam between the 2 cross-staff
notes slopes slightly upwards.  If I add a dynamic, say \f , after <c'
g>2. in line 7, the staves are forced further apart, and the beam
slopes slightly downwards.

If I now add a further dynamic, say ^\p , to the lower staff after <e
c>2. in line 9, the staves are forced still further apart and the beam
now has a large downward slope.

The problem I have is that, in "real life", I don't know how far apart
the staves will be because of the space requirements of other parts of
the music.  Generally I would prefer to allow Lilypond to determine
where line ends etc. should be, so I don't even know which bits of the
music will affect the slope of the beam.  And even if I did know, an
alteration to the music in the closely preceding and following bars
might well affect the spacing of the staves and hence the slope of the
beam.

Is there any way of ensuring that the beam always has a predetermined
slope - say 10° downwards - and that the stem lengths will
automatically be adjusted to allow for this?

David

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