// f: func(Lisp) -> Lisp; may return null
static Lisp lispMap_after(Lisp l, O f) {
  if (l == null) null;
  Lisp l2 = lisp(l.head);
  for (Lisp child : l)
    l2.add(lispMap_after(child, f));
  ret (Lisp) callPostProcessor2(f, l2);
}