Phrasing slur over polyphonic melodies

[Raphael Slinckx]

Hi !

I've a little problem I can't figure out, and I hope someone here knows 
the answer !

Imagine somthing like this (where numbers represent notes):

<< {1 2 3} \\
   {4 5 6} >>

(Here we have a bar break)

<< {7 8 9} \\
   {a b c} >>

Now I want a phrasing slur between notes 1 and 9 , when I try, lilypond 
throws me warning because it can't find beginning of slur and end of 
slur, so I suppose this is illegal !

How is it possible to achieve that result ?

Thanks !
Raphael