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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 addModel(self, moduleName, modelName, model):
 """ Add a model instance to the application model pool. :param moduleName: <str> module name in which the model is located :param modelName: <str> model name :param model: <object> model instance :return: <void> """

        modelIdentifier = "{}.{}".format(moduleName, modelName)
        if modelIdentifier not in self._models:
            self._models[modelIdentifier] = model
        else:
            message = "Application - addModel() - " \
                      "A model with the identifier {} already exists." \
                      .format(modelIdentifier)
            raise Exception(message)