static GazelleTree gazelle_searchTreeUpwards(F1<GazelleTree, Bool> pred, GazelleTree tree) {
  if (tree == null || tree.parent == null) null;
  L<GazelleTree> l = tree.parent.children;
  if (!tree.parent.isSplitNode) l = reversed(l);
  int i = indexOf(l, tree);
  ret gazelle_searchTree_breadthFirst_backwards(pred, takeFirst(l, i));
}