clefs not shown

[Stefan Thomas]

Dear Lilypond users,
why are the clefs not shown in the below quoted example?

rechts = { \clef "treble" s1*2 \break s1*2 }
links = { \clef bass s1*4 }

\version "2.11.43"
\score {
\new PianoStaff = "Klavier" 
        << 
        \new Staff= "rechts"
        \with { \remove "Time_signature_engraver" \remove "Bar_number_engraver" }  
        { \rechts }
           \new Staff = "links" \with {
          \remove "Time_signature_engraver" \remove "Bar_number_engraver" }  
    { \links} >>    


}
 
     \layout {
       \context {
         \Score
         \remove "Bar_number_engraver"
       }
       \context  { \Staff \remove Bar_engraver } 
     }