dynamic line spanner overrides and voice contexts

[James E. Bailey]

Okay, it took me a while to figure all of this out. Normally, with the  
piano centered dynamics needing to be attached to a voice, I'd just  
name the voice and be done with it, however, when I do that, with the  
staff changes, I encounter another problem: the dynamics follow the  
voice into the lower staff.
\version "2.11.57"
\layout {
   \context {
      \type "Engraver_group"
      \name Dynamics
      \alias Voice
      \consists "Output_property_engraver"

      \override VerticalAxisGroup #'minimum-Y-extent = #'(-1 . 1)
      \override DynamicLineSpanner #'Y-offset = #0

      \consists "Script_engraver"
      \consists "Dynamic_engraver"
      \consists "Text_engraver"

      \consists "Skip_event_swallow_translator"

      \consists "Axis_group_engraver"
   }
   \context { \GrandStaff \accepts Dynamics }
}

musici = \relative c'' { d2 d, \change Staff = "musicii" \voiceOne d2  
d, d' d, \change Staff = "musici" d'2 d' d, d' \change Staff =  
"musicii" d, d, d' d,}
musicii = \relative c{ \clef bass R1 \voiceTwo c2 b a g c d e f c b a g}
dynamics = { \override DynamicTextSpanner #'dash-period = #-1.0  
\crescTextCresc s1\mf s1\p s1\< s1 s1\!\mf s1\p }

\score {
   <<
      \new GrandStaff <<
         \new Staff = musici << \musici >> \new Dynamics \dynamics  
\new Staff = musicii << \musicii >>
      >>
   >>
}
\score {
   <<
      \new GrandStaff <<
         \new Staff = musici << \context Voice = "musici" \musici >>  
\new Dynamics << \context Voice = "musici" \dynamics >> \new Staff =  
musicii << \musicii >>
      >>
   >>
}