Reconnecting voices in Lilypond

[darius]

Hi, 

I just wondered how I could manage to put a slur between the
a and the c in a fragment such as:

{
  <
     a4
    \\
     b4 
  >

  < 
    c4
    \\
    d4
  >
}

I understand of course that by restructuring the fragment as
< 
  { a4 ( c4)}
\\
  {b4 d4}
>

the problem would be solved, but I need to add 
dozens of slurs to a score where the construct as described above
occurs all the time, so restructuring it entirely is not really an
option.

On the other hand, the < \\ > are not written manually, they are
generated by the expansion of M4 macros, so that a solution which
would connect the upper and lower voices of adjacent parralel blocks
would do the trick, no matter how arcane the syntax: it would have to 
happen in the macro definition only.


I use Cygwin 2.2.2.

Thank you !

Darius.