code: func (itr *sliceIterator) Next() (rowID, columnID uint64, eof bool) {
  	if itr.i >= itr.n {
  		return 0, 0, true
  	}
  
  	rowID = itr.rowIDs[itr.i]
  	columnID = itr.columnIDs[itr.i]
  
  	itr.i++
  	return rowID, columnID, false
  }
docs: // Next returns the next row/column ID pair.