'%s' is not iterable.

Raise code

            iters = []
            for i, s in enumerate(self.sets):
                iters.append(iproduct(s, {Integer(i)}))

            return iter(roundrobin(*iters))
        else:
            raise ValueError("'%s' is not iterable." % self)

    def __len__(self):
        """
        Returns the length of the disjoint union, i.e., the number of elements in the set.

        Examples
        ======== """