def new_edge(self, node_a, node_b, cost=<NUM_LIT:1>):
edge_id = super(UndirectedGraph, self).new_edge(node_a, node_b, cost)<EOL>self.nodes[node_b]['<STR_LIT>'].append(edge_id)<EOL>return edge_id<EOL>
Adds a new, undirected edge between node_a and node_b with a cost.
        Returns the edge id of the new edge.