new_contents: from .exceptions import UndefinedOperatorError
  
  
  def operator_is_defined(op):
      def checker(operator_function):
          def do_check(self, other):
              if op not in self._operators:
                  raise UndefinedOperatorError(op, self, other, self._operators)
              else:
                  return operator_function(self, other)
          return do_check
      return checker
  
subject: Add check module