%% http://lsr.di.unimi.it/LSR/Item?id=308
%% => http://lilypond.1069038.n5.nabble.com/LSR-Dashed-slurs-indicating-optional-slurs-between-lyric-lines-tc168671.html

\score {
  <<
    \new Voice = "melody" \relative c' {
      \set melismaBusyProperties = #'()
      \slurDashed
      c8 e d-\tweak line-thickness #2.5 ( f ) e g f a
    }
    \new Lyrics \lyricsto "melody" {
      One two three four five six seven eight
    }
    \new Lyrics \lyricsto "melody" {
      One two Whee! \skip 8 that's a dashed slur!
    }
  >>
}