[basic question] how to disable text spanners for the lower staff of a PianoStaff

[James Harkins]

I've got a piano staff (for guzheng, just notating on two staves), and an 
"accel." text spanner in a \global var.

The accel is printed in both staves of the piano staff.

I'd thought this would hide it from the lower staff:

  \new PianoStaff \with {
    instrumentName = "古筝"
    shortInstrumentName = "古筝"
  } <<
    \new Staff = "up" << \global \zhengR >>
    \new Staff = "down" \with { \remove Text_spanner_engraver }
    << \clef bass \global \zhengL >>
  >>

But it still appears.

Some magic incantation I'm missing... can anyone advise? Thanks.

hjh