docs: def add_opinion(self,opinion_obj):
          """
          Adds an opinion to the opinion layer
          @type opinion_obj: L{Copinion}
          @param opinion_obj: the opinion object
          """
          if self.opinion_layer is None:
              self.opinion_layer = Copinions()
              self.root.append(self.opinion_layer.get_node())
          self.opinion_layer.add_opinion(opinion_obj)