Comment #12 on issue 2584 by [email protected]: please make partcombine merge slurs
http://code.google.com/p/lilypond/issues/detail?id=2584
LilyPond does not try to support multiple ending events with the same id, because LilyPond does not support concurrent slurs. The only exception is appoggiatura slurs, which Reinhold gave a distinctive spanner-id.
If you want to be more intelligent about c4(( in manual input then I say the deficiency is in the parser, creating two start-events with default spanner-ids that thus refer to the same slur.
