Notation: Cross-staff kneed beams

A cross-staff beam is a beam which begins on notes in
one staff, and extends through the gap between the
staves onto notes in another neighbouring staff. A
kneed beam is a beam that connects notes with a mixture
of up stems and down stems.

Here are some examples of cross-staff kneed beams in
Lilypond:

http://www.lilypond.org/doc/v2.5/input/regression/out-www/collated-files.html#beam-cross-staff-slope.ly

Notes for future implementation:

Auto-kneeing is the process of deciding whether to use
a kneed beam. If the slope of a beam between two
note-heads, which may be either on the same stave or on
two neighbouring staves and which are to be beamed
together, exceeds the maximum allowable slope threshold
then:

1) a kneed beam is used with its midpoint at a
  height corresponding to the vertical halfway
  point between the two note-heads, and

2) the stems of the notes on different staves are
  made to face in opposite directions in order to
  minimise the slope of the kneed beam.