\RemoveEmptyStaffContext (?)

[PMA]

Hi List.

I have a 40-bar LilyPond piano score notated in
3-staff systems (to accomodate a leaping-over
left hand).  Occasionally one staff in a system,
either the middle or upper, contains rests only.

I want to exclude such staves from the output,
and so have defined my \score block as follows,
following Notation & Snippets Ref. suggestions.

\score {
  \new PianoStaff <<
    \set PianoStaff.instrumentName = #""
    \new Staff = "upper" \upper
    \new Staff = "theme" \theme
    \new Staff = "lower" \lower
  >>
  \layout {
    \context { \RemoveEmptyStaffContext }
  }
}

The output shows no errors, but also no effect:
its rest-only staves are still there.  Could some-
one possibly point up what I've overlooked?

Thanks,
Pete

P.S.  My LilyPond version is 2.13.32.  Also,
        "GrandStaff" yields the same result.