code: def set_value(self, pymux, value):
          
          try:
              value = int(value)
              if value < 0:
                  raise ValueError
          except ValueError:
              raise SetOptionError('Expecting an integer.')
          else:
              setattr(pymux, self.attribute_name, value)
docs: Take a string, and return an integer. Raise SetOptionError when the
          given text does not parse to a positive integer.