code: def add_predicate(self, predicate_obj):
          
          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)
docs: Adds a predicate to the semantic layer
          @type predicate_obj: L{Cpredicate}
          @param predicate_obj: the predicate object