class TreeOfListFollowups<A> extends Tree<A> {
  L<A> list;
  MultiMap<A, Int> index;
  
  *(L<A> *list, MultiMap<A, Int> *index, A a) { super(a); }

  L<Tree<>> children() {
    L<Int> indices = index.get(a);
    ret children = map(entriesFollowingIndices(list, indices),
      s -> new TreeOfListFollowups<A>(list, index, s));
  }
}