code: def _validate_oneof(self, definitions, field, value):
          
          valids, _errors = \
              self.__validate_logical('oneof', definitions, field, value)
          if valids != 1:
              self._error(field, errors.ONEOF, _errors,
                          valids, len(definitions))
docs: {'type': 'list', 'logical': 'oneof'}