src/vs/workbench/contrib/notebook/browser/viewModel/notebookViewModelImpl.ts/NotebookViewModel/undo
class NotebookViewModel:	async undo() {

		const editStack = this._undoService.getElements(this.uri);
		const element = editStack.past.length ? editStack.past[editStack.past.length - 1] : undefined;

		if (element && element instanceof SingleModelEditStackElement || element instanceof MultiModelEditStackElement) {
			await this._withElement(element, async () => {
				await this._undoService.undo(this.uri);
			});

			return (element instanceof SingleModelEditStackElement) ? [element.resource] : element.resources;
		}

		await this._undoService.undo(this.uri);
		return [];
	}