Bug: Crash on {} => true

[jeswr]

The following causes a crash

import { Parser } from 'n3';
import { write } from '@jeswr/pretty-turtle';

const parser = new Parser({ format: 'text/n3' });
write(parser.parse('{} => true .'), { format: 'text/n3' }).then(console.log)

[jeswr]

Related to rdfjs/N3.js#328 (comment)