Save to file.

        Parameters
        ----------
        fp : `file` or `str`, optional
            Output file (default: stdout).

def do_save(self, fp=None):
        """Save to file.

        Parameters
        ----------
        fp : `file` or `str`, optional
            Output file (default: stdout).
        """
        if fp is None:
            fp = sys.stdout
        data = {
            'settings': self.settings,
            'nodes': self.nodes,
            'backward': self.backward
        }
        json.dump(data, fp, ensure_ascii=False)