def METHOD_NAME(self, var_mapping):
    """
    Returns the right hand side of an algebraic equation satisfied by
    this species. This is a utility function called by the
    algebraic_equation_system method.
    EXAMPLES::
        sage: X = species.SingletonSpecies()
        sage: S = X + X
        sage: S.algebraic_equation_system()
        [node1 + (-2*z)]
    """
    return sum(var_mapping[operand] for operand in self._state_info)