\version "2.17.95"

form = \new Staff \with {
  \remove "Time_signature_engraver"
  \remove "Clef_engraver"
  \remove "Bar_engraver"
  \remove "Staff_symbol_engraver"
  \remove "Axis_group_engraver"
} {

  \mark \markup \box \bold "Intro"
  s1 * 8
  \mark \default  % A section
  \repeat volta 2 {
    s1 * 6
  } \alternative {
    { % first ending
      s1 * 2
    }
    { % second ending
      s1 * 2
    }
  } % repeat
 
  \mark \default
  s1 * 8 % B section

  \mark \default
  s1 * 8 % C section

  \bar "|."

} % form

melody = \relative c'' {

  c,4 c c c
  d4 d d d
  e4 e e e
  f4 f f f
  g4 g g g
  a4 a a a
  b4 b b b
  c4 c c c

  c,4 c c c
  d4 d d d
  e4 e e e
  f4 f f f
  g4 g g g
  a4 a a a

% first ending:
  b4 b b b
  c4 c c c

% second ending:
  c,4 c c c
  d4 d d d

% B section:
  e4 e e e
  f4 f f f
  g4 g g g
  a4 a a a
  b4 b b b
  c4 c c c
  c,4 c c c
  d4 d d d

% C section:
  e4 e e e
  f4 f f f
  g4 g g g
  a4 a a a
  b4 b b b
  c4 c c c
  c,4 c c c
  d4 d d d

} % melody

\score {
  <<
    \set Score.markFormatter = #format-mark-box-letters
    \form
    % \melody
    % s1*15 % crash
    s1*16 % no crash
  >>
} % score