// -1 if no matching leaf found, otherwise number of steps to get there
static int walkStringTreeToLeaf(StringTree1 tree, L<S> items) {
  int i = 0;
  while (tree != null && i < l(items)) {
    if (tree.leaf)
      ret i;
    tree = tree.children.get(items.get(i));
    ++i;
  }
  ret -1;
}