Repeating cross staff tremolo

[Allen Wu]

Hey Guys,

I was trying the example of crossing staff tremolo, but I found it difficult to repeat the same one to the next.  Can someone tell me what did I do wrong?  

Thanks,

Allen

%Here is my code:

upper = {s4 s4}

lower = \relative c''' {
  \voiceOne
  \repeat tremolo 8 {<g d'>32 \change Staff = "up" \voiceTwo <af ef'>} %Works

  \voiceOne \change Staff = "down"
  \repeat tremolo 8 {<g d'>32 \change Staff = "up" \voiceTwo <af ef'>}  %Wrong
}


\new PianoStaff <<
  \new Staff = "up"   \upper
  \new Staff = "down" \lower
>>