#4652 partcombine problem with double explicit beams and slurs

[Simon Albrecht]

Reported by Gilberto Agostinho:

When two parts contain manual beams and slurs at the same point and one uses \partcombine, some warnings are outputs in the console: "unterminated beam" and "unterminated slur". It does output the music correctly though.

:::Tex
\version "2.19.22"
{
  \partcombine
  {r4 c'8[( d' e' f' g' a'])}
  {r4 c'8[( b c' d' e' f'])}
}

The problem arose somewhere between 2.19.20 and 2.19.22. The log output from 2.19.22 reads

Starting lilypond 2.19.22 [Untitled]...
Processing `/tmp/frescobaldi-bRFczm/tmp1PBfw2/document.ly'
Parsing...
Interpreting music...
Interpreting music...
Interpreting music...
/tmp/frescobaldi-bRFczm/tmp1PBfw2/document.ly:5:10: warning: unterminated beam
  {r4 c'8
         [( b c' d' e' f'])}
/tmp/frescobaldi-bRFczm/tmp1PBfw2/document.ly:5:11: warning: unterminated slur
  {r4 c'8[
          ( b c' d' e' f'])}
Preprocessing graphical objects...
Finding the ideal number of pages...
Fitting music on 1 page...
Drawing systems...
Layout output to `/tmp/lilypond-a5SBNA'...
Converting to `document.pdf'...
Deleting `/tmp/lilypond-a5SBNA'...
Success: compilation successfully completed
Completed successfully in 0.8".