New in version 2.6.
True when expr is a sequence of Abjad spanners:
abjad> voice = Voice("c'8 d'8 e'8 f'8")
abjad> spanner = spannertools.BeamSpanner(voice[:2])
abjad> spannertools.all_are_spanners([spanner])
True
True when expr is an empty sequence:
abjad> spannertools.all_are_spanners([])
True
Otherwise false:
abjad> spannertools.all_are_spanners('foo')
False
Return boolean.