src/vs/workbench/contrib/files/common/explorerFileNestingTrie.ts/PreTrie/toString
class PreTrie:	toString(indentation = ''): string {
		const lines = [];
		if (this.value.hasItems) {
			lines.push('* => \n' + this.value.toString(indentation + '  '));
		}
		[...this.map.entries()].map(([key, trie]) =>
			lines.push('^' + key + ' => \n' + trie.toString(indentation + '  ')));
		return lines.map(l => indentation + l).join('\n');
	}