Kneed beams in single staff polyphony

[mari+lilypond]

Hi,

the following example shows three bars with should all show the eighth
notes as in bar 1 with kneed beams. But I could not get it working in
single staff polyphony environment (bar 2 and 3), neither automatically
nor manually. Can someone give me a hint please what to do.

Thanks,
Mari


\version "2.19.65"

global = {
  \key c \major
  \time 4/4
}

notes = {
  c8 \change Staff="right" c'' \change Staff="left" c,, \change
Staff="right" c'' \change Staff="left" c,,2
}

right = \relative c'' {
  \global
  s1 s1 s1
}

left = \relative c {
  \global

  \notes

  << { \notes } \\ { a2 a2 } >>

  << { \override Beam.auto-knee-gap = #1
       \notes } \\ { a2 a2 } >>
}

\score {
  \new PianoStaff \with {
    instrumentName = "Piano" }
  <<
    \new Staff = "right" \with {
    } \right
    \new Staff = "left" \with {
    } { \clef bass \left }
  >>
  \layout { }
  \midi { }
}