strange stems and beams

[Stefan Thomas]

Dear lilypond-users,
I wonder, why the stems and beeams are looking so strange in the below quoted example:

re = { \change Staff = rechts }
li = { \change Staff = links }

%begin%%%
rechts = \relative c'' { \time 6/8 < ees ees'>8 [ r8 \li <d d'> ] \re < ees ees'> [ r8 \li <c c'> ] }
links = { s2. }
\score {
\new PianoStaff = "Klavier" 
        << 
        \new Staff= "rechts" { \rechts }
           \new Staff = "links" \with {
      \override VerticalAxisGroup #'keep-fixed-while-stretching = ##t }
    { \links} >>    
}
%%% end%%%