code: def add_constituency_tree(self,my_tree):
          
          if self.constituency_layer is None:
              self.constituency_layer = Cconstituency()
              self.root.append(self.constituency_layer.get_node())
          self.constituency_layer.add_tree(my_tree)
docs: Adds a constituency tree to the constituency layer
          @type my_tree: L{Ctree}
          @param my_tree: the constituency tree object