src/vs/workbench/contrib/notebook/browser/contrib/fold/folding.ts/FoldingController/setFoldingStateDown
class FoldingController:	setFoldingStateDown(index: number, state: CellFoldingState, levels: number) {
		const doCollapse = state === CellFoldingState.Collapsed;
		let region = this._foldingModel!.getRegionAtLine(index + 1);
		let regions: FoldingRegion[] = [];
		if (region) {
			if (region.isCollapsed !== doCollapse) {
				regions.push(region);
			}
			if (levels > 1) {
				let regionsInside = this._foldingModel!.getRegionsInside(region, (r, level: number) => r.isCollapsed !== doCollapse && level < levels);
				regions.push(...regionsInside);
			}
		}

		regions.forEach(r => this._foldingModel!.setCollapsed(r.regionIndex, state === CellFoldingState.Collapsed));
		this._updateEditorFoldingRanges();
	}