Remove an edge from the graph.

        @type  edge: tuple
        @param edge: Edge.

def del_edge(self, edge):
        """
        Remove an edge from the graph.

        @type  edge: tuple
        @param edge: Edge.
        """
        u, v = edge
        self.node_neighbors[u].remove(v)
        self.del_edge_labeling((u, v))  
        if (u != v):
            self.node_neighbors[v].remove(u)
            self.del_edge_labeling((v, u))