src/vs/editor/contrib/documentSymbols/browser/outlineModel.ts/OutlineModel/getTopLevelSymbols
class OutlineModel:	getTopLevelSymbols(): DocumentSymbol[] {
		const roots: DocumentSymbol[] = [];
		for (const child of this.children.values()) {
			if (child instanceof OutlineElement) {
				roots.push(child.symbol);
			} else {
				roots.push(...Iterable.map(child.children.values(), child => child.symbol));
			}
		}
		return roots.sort((a, b) => Range.compareRangesUsingStarts(a.range, b.range));
	}