code: def _L(dfs_data):
      
      """L(T) = {v | the first w in Adj[v] corresponds to a frond vw}."""
      node_set = set()
      for v, adj in list(dfs_data['adj'].items()):
          w = adj[0]
          if is_frond(v, w, dfs_data):
              node_set.add(v)
      return list(node_set)
docs: L(T) contains leaves and branch points for the DFS-tree T.