L(T) contains leaves and branch points for the DFS-tree T.

def _L(dfs_data):
    """L(T) contains leaves and branch points for the DFS-tree T."""
    """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)