docs: public void addUUID( long lo, long hi ) {
      if( _ls==null || _ds== null || _sparseLen >= _ls.length )
        append2slowUUID();
      _ls[_sparseLen] = lo;
      _ds[_sparseLen++] = Double.longBitsToDouble(hi);
      _len++;
      assert _sparseLen <= _len;
    }