def stem(u, v, dfs_data):
<EOL>uv_edges = dfs_data['<STR_LIT>'].get_edge_ids_by_node_ids(u, v)<EOL>buv_edges = B(u, v, dfs_data)<EOL>for edge_id in uv_edges:<EOL><INDENT>if edge_id in buv_edges:<EOL><INDENT>return edge_id<EOL><DEDENT><DEDENT>return None<EOL>
The stem of Bu(v) is the edge uv in Bu(v).