docs: public Key toKey() {
      AutoBuffer bs = new AutoBuffer();
      bs.put4(_data_id);
      bs.put8(_seed);
      bs.put1(_producerId);
      _tree.write(bs);
      Key key = Key.make((byte)1,Key.DFJ_INTERNAL_USER, H2O.SELF);
      DKV.put(key,new Value(key, bs.buf()));
      return key;
    }