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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 document(self, document_id=None):
 """Create a sub-document underneath the current collection. Args: document_id (Optional[str]):
 The document identifier within the current collection. If not provided, will default to a random 20 character string composed of digits, uppercase and lowercase and letters. Returns: ~.firestore_v1beta1.document.DocumentReference: The child document. """

        if document_id is None:
            document_id = _auto_id()

        child_path = self._path + (document_id,)
        return self._client.document(*child_path)