static GazelleTree gazelle_searchTreeUpwards(F1<GazelleTree, Bool> pred, GazelleTree tree) {
  if (tree == null) null;
  L<GazelleTree> l = tree.parent.children;
  int i = indexOf(l, tree);
  for (int j = i-1; j >= 0; j--) {
    GazelleTree t =  gazelle_searchTree(pred, l.get(j));
    if (t != null) ret t;
  }
  null;
}