code: def del_edge(self, 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))
docs: Remove an edge from the graph.
  
          @type  edge: tuple
          @param edge: Edge.