code: def is_multiple_of(ref):
      
      def is_multiple_of_ref(x):
          if x % ref == 0:
              return True
          else:
              raise IsNotMultipleOf(wrong_value=x, ref=ref)
              # raise Failure('x % {ref} == 0 does not hold for x={val}'.format(ref=ref, val=x))
  
      is_multiple_of_ref.__name__ = 'is_multiple_of_{}'.format(ref)
      return is_multiple_of_ref
docs: Validates that x is a multiple of the reference (`x % ref == 0`)