static LL<WebTree> webTrees_walkInParallel(L<WebTree> trees) {
  new LL<WebTree> out;
  webTrees_walkInParallel_impl(trees, out);
  ret out;
}

static void webTrees_walkInParallel_impl(L<WebTree> trees, LL<WebTree> out) {
  out.add(trees);
  trees = cloneList(trees);
  int n = 0;
  for (WebTree t : trees) if (t != null) n = max(n, l(t.children));
  for i to n: {
    new L<WebTree> children;
    for (WebTree t : trees)
      children.add(t == null ? null : pairB(get(t.children, i)));
    webTrees_walkInParallel_impl(children, out);
  }
}