Adds a predicate to the semantic layer
        @type predicate_obj: L{Cpredicate}
        @param predicate_obj: the predicate object

def add_predicate(self, predicate_obj):
        """
        Adds a predicate to the semantic layer
        @type predicate_obj: L{Cpredicate}
        @param predicate_obj: the predicate object
        """
        if self.srl_layer is None:
            self.srl_layer = Csrl()
            self.root.append(self.srl_layer.get_node())
        self.srl_layer.add_predicate(predicate_obj)