static <A> Pair<A, Int> stringTreeLeafValue2(StringTree2<A> tree, L<S> items) {
  int i = 0;
  while (tree != null && i < l(items)) {
    if (tree.leafValue != null)
      ret pair(tree.leafValue, i);
    tree = tree.children.get(items.get(i));
    ++i;
  }
  null;
}