New in version 2.6.
True when expr is a sequence of Abjad duration tokens:
abjad> from abjad.tools import durationtools
abjad> duration_tokens = ['8.', (3, 16), Fraction(3, 16), Duration(3, 16)]
abjad> durationtools.all_are_duration_tokens(duration_tokens)
True
True when expr is an empty sequence:
abjad> durationtools.all_are_duration_tokens([])
True
Otherwise false:
abjad> durationtools.all_are_durations('foo')
False
Return boolean.