src/vs/base/browser/ui/tree/indexTreeModel.ts/IndexTreeModel/getParentNodeLocation
class IndexTreeModel:	getParentNodeLocation(location: number[]): number[] | undefined {
		if (location.length === 0) {
			return undefined;
		} else if (location.length === 1) {
			return [];
		} else {
			return tail2(location)[0];
		}
	}