@Override
 public OperationTree read(Kryo kryo, Input input, Class<OperationTree> type) {
   return util.toNode((Operation) kryo.readClassAndObject(input));
 }
 @Override
 public void write(Kryo kryo, Output output, OperationTree node) {
   kryo.writeClassAndObject(output, util.getOperation(node));
 }