code: def _is_valid_position(self, position):
          '''
          check if given position is valid for this collection
          '''
          row, col = position
          valid_r = row in self.row_labels
          valid_c = col in self.col_labels
          return valid_r and valid_f _is_valid_position(self, position):
          '''
          check if given position is valid for this collection
          '''
          row, col = position
          valid_r = row in self.row_labels
          valid_c = col in self.col_labels
          return valid_r and valid_c
docs: check if given position is valid for this collection