code: public void addUUID( long lo, long hi ) {
      if (C16Chunk.isNA(lo, hi)) {
        addNA();
        return;
      }
      if( _ms==null || _ds== null || _sparseLen >= _ms.len() )
        append2slowUUID();
      _ms.set(_sparseLen,lo);
      _ds[_sparseLen] = Double.longBitsToDouble(hi);
      if (_id != null) _id[_sparseLen] = _len;
      _sparseLen++;
      _len++;
      assert _sparseLen <= _len;
    }
docs: Append a UUID, stored in _ls & _ds