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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 get_existing_model(model_name):
 """ Try to find existing model class named `model_name`. :param model_name: String name of the model class. """

    try:
        model_cls = engine.get_document_cls(model_name)
        log.debug('Model `{}` already exists. Using existing one'.format(
            model_name))
        return model_cls
    except ValueError:
        log.debug('Model `{}` does not exist'.format(model_name))