code: public static <Head, Tail extends HList> Lens.Simple<HCons<Head, ? extends Tail>, Tail> tail() {
          return simpleLens(HCons::tail, (hCons, newTail) -> cons(hCons.head(), newTail));
      }
docs: Focus on the tail of an {@link HList}.
  
  @param <Head> the head element type
  @param <Tail> the tail HList type
  @return a lens that focuses on the tail of an HList