def add_constituency_tree(self,my_tree):
        """
        Adds a constituency tree to the constituency layer
        @type my_tree: L{Ctree}
        @param my_tree: the constituency tree object
        """
        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)
Adds a constituency tree to the constituency layer
        @type my_tree: L{Ctree}
        @param my_tree: the constituency tree object