docs: def natural(value, argument='argument'):
      """ Restrict input type to the natural numbers (0, 1, 2, 3...) """
      value = _get_integer(value)
      if value < 0:
          error = ('Invalid {arg}: {value}. {arg} must be a non-negative '
                   'integer'.format(arg=argument, value=value))
          raise ValueError(error)
      return value