Suppressing staff

[Helge Kruse]

Hello,

I have a piece for singer and piano. I want to write the notes for the 
piano. The words should be written above the piano notes as the would 
appear when the singer's staff is visible. But since the piano melody is 
mostly the same as the singer's melody the singers staff should be 
hidden in most lines.
Two measures are different. Here I would like to write print the 
singer's notes.

How can I suppress the singers staff but keep the words above the piano 
staves?

\version "2.16.0"

common = { \key g\major \time 3/4 }
words = \lyricmode {
  La, la la, la. }
singer = \relative c'' {
  \common
  b a g fis }
pianoRight = \relative c'' {
  \common
<< {b a g fis } \\ { <e c>2. } >>  }
pianoLeft = \relative c {
  \common
  \clef bass d4. d8 d,4 g2 }

\score {
<<
    \new Staff { <<  \new Voice = "one" \singer >> }
    \new Lyrics \lyricsto "one" { \words }
    \new PianoStaff <<
      \new Staff { <<  \pianoRight >> }
      \new Staff { <<  \pianoLeft >> }
>>
>>
}