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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def addService(self, moduleName, serviceName, service):
 """ Add a service instance to the application service pool. :param moduleName: <str> module name in which the service is located :param serviceName: <str> service name :param service: <object> service instance :return: <void> """

        serviceIdentifier = "{}.{}".format(moduleName, serviceName)
        if serviceIdentifier not in self._services:
            self._services[serviceIdentifier] = service
        else:
            message = "Application - addService() - " \
                      "A service with the identifier {} already exists." \
                      .format(serviceIdentifier)
            raise Exception(message)