Hiding/unhiding empty staves

[Paolo Prete]

Hello,

The following snippet hides "empty" staves in a system. But how can I unset
this behavior for selected staves?
For example: I want to hide the empty staff in the second measure but in
the fourth measure I want to print the staff with its rest...

Thanks for your help!

----------------------------------------------------

upper  =  {

 c'1 \break
 r \break
 c' \break
 r

}

middle =  { d'1 d' d' d'}
lower  =  { e'1 e' e' d'}


\score {
  \new PianoStaff <<
    \new Staff = "upper" \upper
    \new Staff = "middle" \middle
    \new Staff = "lower" \lower
  >>
  \layout {
    \context {
      \Staff \RemoveEmptyStaves
    }
    \context {
      \PianoStaff
      \remove "Keep_alive_together_engraver"
    }
  }
}