private void reset(int op) {
    done();
    _buffer.reset();
    set(_buffer);

    _id = ID.getAndIncrement();

    writeInt(0); // length: will set this later
    writeInt(_id);
    writeInt(0); // response to
    writeInt(op);
  }
 OutMessage(Mongo m) {
   _mongo = m;
   _buffer = _mongo == null ? new PoolOutputBuffer() : _mongo._bufferPool.get();
   set(_buffer);
 }