detecting the start and end of a polyphonic passage from scheme

[Maurits Lamers]

Hi all,

I am trying to build a system based on the listener system which can identify 
voices in a piece of music.
I use a listener to the Voice context to give me the note events.

In the following passage I can use (ly:context-id (ly:translator-context 
engraver) to get the id, which is empty for c4,
"1" for the first voice, "2" for the second voice, and empty again for d4 at 
the end.

time 4/4 \relative c' {  c4 << { c8 d e4 } \\ { c8 b c4 } >> d4 | }


However in the following passage the context-id cannot be used because it is 
not set.

\relative c' {
      \new Voice {
        c4
        <<
          \new Voice {
            c8 d e4
          }
          \new Voice {
            c8 b c4
          }
        >>
        d4
      }
    }

I am currently using object properties to follow these voices by setting a 
unique value for that voice context object, but that doesn't help me
to detect properly the start and end of the polyphonic passage. The main voice 
gets 1, the first nested voice will be 2, the second nested voice will be 3.
For transcription purposes it would be very helpful to detect the start and end 
of the polyphonic passage. Is this possible?

cheers

Maurits