New in version 2.0.
Get exactly one LilyPond command mark attached to component:
abjad> staff = Staff("c'8 d'8 e'8 f'8")
abjad> marktools.LilyPondCommandMark('stemUp')(staff[0])
LilyPondCommandMark('stemUp')(c'8)
abjad> f(staff)
\new Staff {
\stemUp
c'8
d'8
e'8
f'8
}
abjad> marktools.get_lilypond_command_mark_attached_to_component(staff[0])
LilyPondCommandMark('stemUp')(c'8)
Return one LilyPond command mark.
Raise missing mark error when no LilyPond command mark is attached.
Raise extra mark error when more than one LilyPond command mark is attached.