static <A, B> L<B> mapWithIndex(L<A> l, IF2<Int, A, B> f) {
  L<B> out = emptyList(l);
  for i over l:
    out.add(f.get(i, l.get(i)));
  ret out;
}

static <A, B> L<B> mapWithIndex(IF2<Int, A, B> f, L<A> l) {
  ret mapWithIndex(l, f);
}