docs: static void appendKey(Key model, final Key tKey, final Key dtKey, final String tString, final int tree_id) {
      final int selfIdx = H2O.SELF.index();
      new TAtomic<SpeeDRFModel>() {
        @Override public SpeeDRFModel atomic(SpeeDRFModel old) {
          if(old == null) return null;
          return SpeeDRFModel.make(old, tKey, dtKey, selfIdx, tString, tree_id);
        }
      }.invoke(model);
    }