Evaluate internal polynomial.

def _poly_eval(self, u, ids, der=0):
        """Evaluate internal polynomial."""
        if der == 0:
            return self._poly_eval_0(u, ids)
        elif der == 1:
            return self._poly_eval_1(u, ids)
        elif der == 2:
            return self._poly_eval_2(u, ids)
        elif der == 3:
            return self._poly_eval_3(u, ids)
        elif der >= 4:
            return self._poly_eval_4(u, ids)
        else:
            raise ValueError("der={} is impossible".format(der))