New in version 2.6.
True when expr is a sequence of Abjad durations:
abjad> from abjad.tools import durationtools
abjad> durations = [Duration((3, 16)), Duration((4, 16))]
abjad> durationtools.all_are_durations(durations)
True
True when expr is an empty sequence:
abjad> durationtools.all_are_durations([])
True
Otherwise false:
abjad> durationtools.all_are_durations('foo')
False
Return boolean.