'This function should only be used internally
self._union(other) returns a new, joined set if self knows how
to join itself with other, otherwise it returns ``None``.
It may also return a python set of SymPy Sets if they are somehow
simpler. If it does this it must be idempotent i.e. the sets returned
must return ``None`` with _union\'ed with each other
Used within the :class:`Union` class'

def _union(self, other):

    return None
