docs: def _validate_cell(self, order, cell):
          """Check that the given cell is valid.
  
          The order is assumed already to have been validated.
          """
  
          max_cells = self._order_num_cells(order)
  
          try:
              cell = int(cell)
          except ValueError as e:
              raise TypeError('MOC cell must be convertable to int')
  
          if not 0 <= cell < max_cells:
              raise ValueError(
                  'MOC cell order {0} must be in range 0-{1}'.format(
                      order, max_cells - 1))
  
          return cell