code: public static <E, List extends HCons<?, ?>> Lens.Simple<List, E> elementAt(Index<E, List> index) {
          return simpleLens(index::get, (l, e) -> index.set(e, l));
      }
docs: Focus invariantly on the element at the specified {@link Index} in an {@link HList}.
  
  @param index  the index of the element to focus on
  @param <E>    the element type
  @param <List> the HList under focus
  @return a lens focusing on the element at index