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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 collections(self):
 """List top-level collections of the client's database. Returns: Sequence[~.firestore_v1beta1.collection.CollectionReference]: iterator of subcollections of the current document. """

        iterator = self._firestore_api.list_collection_ids(
            self._database_string, metadata=self._rpc_metadata
        )
        iterator.client = self
        iterator.item_to_value = _item_to_collection_ref
        return iterator