New in version 2.6.
True when expr is a sequence of Abjad markup:
abjad> markup = markuptools.Markup('some text')
abjad> markuptools.all_are_markup([markup])
True
True when expr is an empty sequence:
abjad> markuptools.all_are_markup([])
True
Otherwise false:
abjad> markuptools.all_are_markup('foo')
False
Return boolean.