crash in 1.7.0

[Hans Kieserman]

The following .ly file crashes with the latest CVS.  (I am attempting to 
create multiple lines of music on one piano staff, and found this crash 
in the process.)


\version "1.7.0"

\score {
        \notes <
                \context Staff = down {
                        \clef bass
                        \context Voice {
                        [ e8 e' ]
                        }
                }
                \context Staff = down <
                        \clef bass
                        \context Voice {
                        c'4
                        }
                >
        >
\paper {}
}