Add a role to the predicate
        @type role_obj: L{Crole}
        @param role_obj: the role object

def add_role(self, role_obj):
        """
        Add a role to the predicate
        @type role_obj: L{Crole}
        @param role_obj: the role object
        """
        role_node = role_obj.get_node()
        self.node.append(role_node)