Input args to Union must be Sets

Raise code

    # ===== Global Rules =====
    if not args:
        return S.EmptySet

    for arg in args:
        if not isinstance(arg, Set):
            raise TypeError("Input args to Union must be Sets")

    # Merge all finite sets
    finite_sets = [x for x in args if x.is_FiniteSet]
    if len(finite_sets) > 1:
        a = (x for set in finite_sets for x in set)
        finite_set = FiniteSet(*a)
        args = [finite_set] + [x for x in args if not x.is_FiniteSet]