code: def XOR(a, b, exc=CertifierValueError('Expected at least one certified value')):
      
      errors = []
  
      for certifier in [a, b]:
          try:
              certifier()
          except CertifierError as e:
              errors.append(e)
  
      if len(errors) != 1:
          if exc is not None:
              raise exc
docs: Only one arg must not raise a Certifier exception when called overall.
      Raise the specified exception on failure.
  
      :params Certifier a:
          The first certifiers to call
      :params Certifier b:
          The second certifiers to call
      :param Exception exc:
          Callable that is raised if XOR fails.