code: def stem(u, v, dfs_data):
      
      #return dfs_data['graph'].get_first_edge_id_by_node_ids(u, v)
      uv_edges = dfs_data['graph'].get_edge_ids_by_node_ids(u, v)
      buv_edges = B(u, v, dfs_data)
      for edge_id in uv_edges:
          if edge_id in buv_edges:
              return edge_id
      return None
docs: The stem of Bu(v) is the edge uv in Bu(v).