docs: private void append2slowUUID() {
      final int CHUNK_SZ = 1 << H2O.LOG_CHK;
      if( _sparseLen > CHUNK_SZ )
        throw new ArrayIndexOutOfBoundsException(_sparseLen);
      if( _ds==null && _ls!=null ) { // This can happen for columns with all NAs and then a UUID
        _xs=null;
        alloc_doubles(_sparseLen);
        Arrays.fill(_ls,C16Chunk._LO_NA);
        Arrays.fill(_ds,Double.longBitsToDouble(C16Chunk._HI_NA));
      }
      if( _ls != null && _ls.length > 0 ) {
        _ls = MemoryManager.arrayCopyOf(_ls,_sparseLen<<1);
        _ds = MemoryManager.arrayCopyOf(_ds,_sparseLen<<1);
      } else {
        alloc_mantissa(4);
        alloc_doubles(4);
      }
      assert _sparseLen == 0 || _ls.length > _sparseLen:"_ls.length = " + _ls.length + ", _sparseLen = " + _sparseLen;
    }