Using slurs with polyphony

[Yuval Harel]

Hello,

I'm having trouble with using slurs where the number of voices changes 
during the slur.
A simple example:
        <<
                g4( \\
                e
        >>
        fis
        <<
                g) \\
                f
        >>
This yields:
        /home/UV/1.ly:5:6: warning: unterminated slur:
                                  g4
                                      ( \\

        /home/UV/1.ly:10:5: warning: can't find start of slur:
                                      g
                                         ) \\
And of course, the slur doesn't show.

I can't tell whether this is a bug, because I couldn't find any 
documentation on
how beginnings and endings of slurs are matched.

So this leaves two questions:
 - How are beginnings and endings of slurs matched?
 - Is there a way to make such slurs work without forcing the number of 
voices to remain constant?

Thanks,
        Yuval