How can I notate this slur with LilyPond?

Question

I have a slur that starts on as multi-voice and ends on a single voice as measure 3–4 of the following:

Here's what I've got so far (right hand only):

\relative c' {
  << { c4 c c c8 c c4 bes8 aes4. aes8 aes } \\
     { g8 aes f g4 aes f8 aes8 g f r8 r2 } >>
  << { bes8 bes2 aes8( g f } \\
     { f8 g ees f4  r r8 } >>
  <f ees>4.) g8 ees des \clef treble
}

With this, I'm getting errors about an unfinished slur. I suspect that this is because the slur starts inside the << … >> scope and tries to cross that border.

Answer 1

Here are two possible solutions.

1: You don't seem to need two voices from aes8 onwards, so use this:

\relative c' {
  << { c4 c c c8 c c4 bes8 aes4. aes8 aes } \\
     { g8 aes f g4 aes f8 aes8 g f r8 r2 } >>
  << { bes8 bes2 } \\ 
     { f8 g ees f4 } >>
     aes8( g f <f ees>4.)
  g8 ees des \clef treble
}

2: You can extend the two-voice stuff longer and use s4. as an "empty" second voice.

\relative c' {
  << { c4 c c c8 c c4 bes8 aes4. aes8 aes } \\
     { g8 aes f g4 aes f8 aes8 g f r8 r2 } >>
  << { bes8 bes2 aes8( g f <f ees>4.) } \\
     { f8 g ees f4  r r8 s4. } >>
  g8 ees des \clef treble
}

Answer 2

You can extend the two-voice stuff longer but you don't need spacer rests in the second voice. The musical expression is as long as the longest voice.

Further the rests in the second voice look unnatural. I would use spacer rests here.

\relative c' {
  << { c4 c c c8 c c4 bes8 aes4. aes8 aes } \\
     { g8( aes f g4) aes f8 aes8( g f s8 \once\hideNotes aes2) } >>
  << { bes8 bes2 \stemDown aes8( g f <f ees>4.) } \\
     { f8( g ees f4) s s8 } >>
  g8( des' f \clef treble as bes)
}

Further it's not always clear where is the slur attached. The slur in question binds notes with stem down what usually designates the second voice. But there are rests below the whole first stave. This calls for redesign.