def METHOD_NAME(self):
    """
    Returns the weight ring for this species. This is determined by
    asking Sage's coercion model what the result is when you add
    elements of the weight rings for each of the operands.
    EXAMPLES::
        sage: S = species.SetSpecies()
        sage: C = S+S
        sage: C.weight_ring()
        Rational Field
    ::
        sage: S = species.SetSpecies(weight=QQ['t'].gen())
        sage: C = S + S
        sage: C.weight_ring()
        Univariate Polynomial Ring in t over Rational Field
    """
    return self._common_parent([self.left_summand().METHOD_NAME(),
                                self.right_summand().METHOD_NAME()])