'This function should only be used internally
self._intersect(other) returns a new, intersected set if self knows how
to intersect itself with other, otherwise it returns ``None``
When making a new set class you can be assured that other will not
be a :class:`Union`, :class:`FiniteSet`, or :class:`EmptySet`
Used within the :class:`Intersection` class'

def _intersect(self, other):

    return None
