code: func (l *List) Walk(walkFunc ListWalkFunc) error {
  	for i := 0; i < len(l.chunks); i++ {
  		chk := l.chunks[i]
  		for j := 0; j < chk.NumRows(); j++ {
  			err := walkFunc(chk.GetRow(j))
  			if err != nil {
  				return errors.Trace(err)
  			}
  		}
  	}
  	return nil
  }
docs: // Walk iterate the list and call walkFunc for each row.