Phrasing slures starting on single voice, ending in multiple voice

[Alberto Simões]

Hi again,

I have this code

 <cis, e a>2 %%% --> this one \(
 <b d g> <a cis fis> ~
 <a d fis> ~
 << { \stemUp g'4 fis e2      <a, d>2 ~ \partial 4 <a d>4}    \\
    { \stemDown d2 ~ d4 cis    s2 s4} \\
    { \stemDown \showStaffSwitch b4 a
          \change Staff = "lower"
      \stemUp g2 fis2~\partial 8 fis4
    }
 >>  %%% Should finish here \)

There was no big deal, if I didn't need a phrasing slur for all this 
portion of music. I tried to open in the right place (check comment)
and make it end at the end of the multiple voice portion. But lilypond 
doesn't like (and I understand why). I tried to make it end in one of 
the voices from the polyphonic portion, but didn't had any success.

What is the better way to do this?
 - put the first 4 chords in a polyphonic notation?
 - or is there any trick?

Thanks
Alberto

--
Alberto Simões - Departamento de Informática - Universidade do Minho
                 Campus de Gualtar - 4710-057 Braga - Portugal