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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 list_documents(self, page_size=None):
 """List all subdocuments of the current collection. Args: page_size (Optional[int]]):
 The maximum number of documents 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.DocumentReference]: iterator of subdocuments of the current collection. If the collection does not exist at the time of `snapshot`, the iterator will be empty """

        parent, _ = self._parent_info()

        iterator = self._client._firestore_api.list_documents(
            parent,
            self.id,
            page_size=page_size,
            show_missing=True,
            metadata=self._client._rpc_metadata,
        )
        iterator.collection = self
        iterator.item_to_value = _item_to_document_ref
        return iterator