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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, page_size=None):
 """List subcollections of the current document. Args: page_size (Optional[int]]):
 The maximum number of collections in each page of results from this request. Non-positive values are ignored. Defaults to a sensible value set by the API. Returns: Sequence[~.firestore_v1beta1.collection.CollectionReference]: iterator of subcollections of the current document. If the document does not exist at the time of `snapshot`, the iterator will be empty """

        iterator = self._client._firestore_api.list_collection_ids(
            self._document_path,
            page_size=page_size,
            metadata=self._client._rpc_metadata,
        )
        iterator.document = self
        iterator.item_to_value = _item_to_collection_ref
        return iterator