produce midi drums but hide staff

[vector]

Hi there,
Im getting the hang of this even tho it might not look it ;) thanks for 
all your inputs.

I wanted to add a simple drumbeat to the previous script and produce a 
basic chord and drums midi.
so far I have this working.
But I want to remove the drums completely from the pdf output. ie just 
show chord progression only.

but
\new DrumStaff <<
      \stopStaff
      \hideNotes
      \new DrumVoice {\voiceOne \up}
      \new DrumVoice { \voiceTwo \down }
   >>
did just that, it hid the staff but the notes are left floating. 
hideNotes had no effect :(
maybe its in the wrong place?