New in version 2.3.
Get key signature mark attached to component:
abjad> staff = Staff("c'8 d'8 e'8 f'8")
abjad> contexttools.KeySignatureMark('c', 'major')(staff)
KeySignatureMark(NamedChromaticPitchClass('c'), Mode('major'))(Staff{4})
abjad> f(staff)
\new Staff {
\key c \major
c'8
d'8
e'8
f'8
}
abjad> contexttools.get_key_signature_mark_attached_to_component(staff)
KeySignatureMark(NamedChromaticPitchClass('c'), Mode('major'))(Staff{4})
Return key signature mark.
Raise missing mark error when no key signature mark attaches to component.