'This function should only be used internally
See Set._union for docstring'

def _union(self, other):

    if other.is_FiniteSet:
        return FiniteSet(*(self._elements | other._elements))
    if any(((sympify(other.contains(x)) is S.true) for x in self)):
        return set((FiniteSet(*[x for x in self if (other.contains(x) != True)]), other))
    return None
