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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_path(self):
 """Create and cache the full path for this document. Of the form: documents/{document_path}`` Returns: str: The full document path. Raises: ValueError: If the current document reference has no ``client``. """

        if self._document_path_internal is None:
            if self._client is None:
                raise ValueError("A document reference requires a `client`.")
            self._document_path_internal = _get_document_path(self._client, self._path)

        return self._document_path_internal