staff switching in custom contexts not working

[Kieren MacMillan]

Hello all,

In the attached snippet, I’ve defined some custom contexts 
(nested/hierarchical) for piano work.
I can’t seem to get staff switching to work correctly: I get “ cannot change 
[…] none of these in my family” errors.
Any hints would be appreciated.

Thanks!
Kieren.

%%%%   BEGIN SNIPPET 

\version "2.19.17"

\layout {
  \context {
    \Staff
    \name PianoSingleStaff
    \alias Staff
  }
  \context {
    \PianoSingleStaff
    \name PianoSingleStaffUpper
    \alias PianoSingleStaff
    \alias Staff
    \override DynamicLineSpanner #'direction = #UP
    \override TextScript #'direction = #UP
  }
  \context {
    \PianoSingleStaff
    \name PianoSingleStaffLower
    \alias PianoSingleStaff
    \alias Staff
    \override DynamicLineSpanner #'direction = #DOWN
    \override TextScript #'direction = #DOWN
  }
  \context {
    \PianoStaff
    \accepts PianoSingleStaffUpper
    \accepts PianoSingleStaffLower
  }
}

pfU = { \change PianoSingleStaff = "piano_upper" \oneVoice }
pfL = { \change PIanoSingleStaff = "piano_lower" \oneVoice }


theMusic = {
  \clef bass
  \pfL d,8 \pfU d' \pfL d, \pfU d'
}

\score {
  \new PianoStaff <<
    \new PianoSingleStaffUpper = "piano_upper" s1
    \new PianoSingleStaffLower = "piano_lower" \theMusic
  >>
}

%%%%   END SNIPPET 

_______________________

Kieren MacMillan, composer
www:  <http://www.kierenmacmillan.info>
email:  address@hidden