code: private void append2slowUUID() {
      if(_id != null)
        cancel_sparse();
      if( _ds==null && _ms!=null ) { // This can happen for columns with all NAs and then a UUID
        _xs=null;
        _ms.switchToLongs();
        _ds = MemoryManager.malloc8d(_sparseLen);
        Arrays.fill(_ms._vals8,C16Chunk._LO_NA);
        Arrays.fill(_ds,Double.longBitsToDouble(C16Chunk._HI_NA));
      }
      if( _ms != null && _sparseLen > 0 ) {
        _ds = MemoryManager.arrayCopyOf(_ds, _sparseLen * 2);
        _ms.resize(_sparseLen*2);
        if(_id != null) _id = Arrays.copyOf(_id,_sparseLen*2);
      } else {
        _ms = new Mantissas(4);
        _xs = null;
        _ms.switchToLongs();
        _ds = new double[4];
      }
    }
docs: Slow-path append data