def _save( self, fn ):
<EOL>j = json.dumps({ '<STR_LIT:description>': self.description(),<EOL>'<STR_LIT>': self._pending,<EOL>'<STR_LIT>': self._results },<EOL>indent = <NUM_LIT:4>,<EOL>cls = MetadataEncoder)<EOL>with open(fn, '<STR_LIT:w>') as f:<EOL><INDENT>f.write(j)<EOL><DEDENT>
Persist the notebook to the given file.

        :param fn: the file name