New in version 2.6.
True when expr is a sequence of Abjad staves:
abjad> staff = Staff("c'4 d'4 e'4 f'4")
abjad> stafftools.all_are_staves([staff])
True
True when expr is an empty sequence:
abjad> stafftools.all_are_staves([])
True
Otherwise false:
abjad> stafftools.all_are_staves('foo')
False
Return boolean.
Function wraps componenttools.all_are_components().