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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:

async def create_app_collections(db):
    ''' load all models in app and create collections in db with specified indices'''
    futures = []
    for model_class in MongoCollectionMixin.__subclasses__():
        if model_class._meta.concrete is True:
            futures.append(create_collection(db, model_class))

    await asyncio.gather(*futures)