code: def walk_rows(self, mapping=identity):
          
          row_in_grid = self._walk.row_in_grid
          return map(lambda row: mapping(row_in_grid(row)), self._rows)
docs: Iterate over rows.
  
          :return: an iterator over :class:`rows <RowsInGrid>`
          :param mapping: funcion to map the result, see
            :meth:`walk_instructions` for an example usage