static <A> L<A> intersperse_dyn(IF2<A> f, Iterable<A> l) {
  new L<A> out;
  A prev = null;
  fOr (A x : l) {
    if (!empty(out))
      out.add(f.get(prev, x));
    out.add(x);
    prev = x;
  }
  ret out;
}