New in version 2.6.
True when expr is a sequence of Abjad containers:
abjad> containers = 3 * Container("c'8 d'8 e'8")
abjad> containertools.all_are_containers(containers)
True
True when expr is an empty sequence:
abjad> containertools.all_are_containers([])
True
Otherwise false:
abjad> containertools.all_are_containers('foo')
False
Return boolean.
Function wraps componenttools.all_are_components().