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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 register(cls):
 """ Register a given model in the registry """

    registry_entry = RegistryEntry(category = cls.category, namespace = cls.namespace, name = cls.name, cls=cls)
    if registry_entry not in registry and not exists_in_registry(cls.category, cls.namespace, cls.name):
        registry.append(registry_entry)
    else:
        log.warn("Class {0} already in registry".format(cls))