First variable cannot be a number: %i

Raise code

        # derivative.
        variable_count = []
        array_likes = (tuple, list, Tuple)

        for i, v in enumerate(variables):
            if isinstance(v, Integer):
                if i == 0:
                    raise ValueError("First variable cannot be a number: %i" % v)
                count = v
                prev, prevcount = variable_count[-1]
                if prevcount != 1:
                    raise TypeError("tuple {} followed by number {}".format((prev, prevcount), v))
                if count == 0:
                    variable_count.pop()
                else: