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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_instance(key, expire=None):
 """Return an instance of RedisSet."""

    global _instances
    try:
        instance = _instances[key]
    except KeyError:
        instance = RedisSet(
            key,
            _redis,
            expire=expire
        )
        _instances[key] = instance

    return instance