\version "2.12.0"
\header {
texidoc = "Concave beaming works for chords as well as monophonic
music.
"
}

morgenliedBeam = \relative c'' \new Voice {
  \time 3/8
  % morgenlied
  c16 b c e g <e b'> |
  \stemUp
  c16 b c e g <e b'> |
}

rachmaninovBeams = \relative c' \new Voice {
  \voiceOne
  \time 4/4
  \key c \minor
  <c' e c'>8[ <g c g'> <c f> <ees, ees'>] ~
  <ees ees'>8[ <c c'> <des des'> <d d'>]
}

horizontalBeams = {
  % todo.
}

nonHorizontalBeams = {
  \morgenliedBeam
  \rachmaninovBeams
}
\layout {
  ragged-right = ##t
}

% cut & paste from beam-concave.ly

#(define (<> x y) (not (= x  y)))
mustBeHorizontal = {
  \override Staff.Beam #'positions =
    #(ly:make-simple-closure
      (ly:make-simple-closure
       (append
        (list chain-grob-member-functions `(,cons 0 0))
        (check-slope-callbacks =))))
}
mustNotBeHorizontal = {
  \override Staff.Beam #'positions =
    #(ly:make-simple-closure
      (ly:make-simple-closure
       (append
        (list chain-grob-member-functions `(,cons 0 0))
        (check-slope-callbacks <>))))
}

\new Voice {
  \mustBeHorizontal
  R1
  \horizontalBeams
  \mustNotBeHorizontal
  \nonHorizontalBeams
}