code: def is_subset(reference_set  # type: Set
                ):
      
      def is_subset_of(x):
          missing = x - reference_set
          if len(missing) == 0:
              return True
          else:
              # raise Failure('is_subset: len(x - reference_set) == 0 does not hold for x=' + str(x)
              #                   + ' and reference_set=' + str(reference_set) + '. x contains unsupported '
              #                      'elements ' + str(missing))
              raise NotSubset(wrong_value=x, reference_set=reference_set, unsupported=missing)
  
      is_subset_of.__name__ = 'is_subset_of_{}'.format(reference_set)
      return is_subset_of
docs: 'Is subset' validation_function generator.
      Returns a validation_function to check that x is a subset of reference_set
  
      :param reference_set: the reference set
      :return: