static <A, B> L<B> lambdaMapLike convertListElementsBothWays(IF1<A, B> fGet, IF1<B, A> fSet, L<A> l) {
  ret new RandomAccessAbstractList<B> {
    final int size = l(l);

    public int size() { ret size; }
    public B get(int i) {
      ret fGet.get(l.get(i));
    }
    public B set(int i, B b) {
      B old = get(i);
      l.set(i, fSet.get(b));
      ret old;
    }
  };
}

Began life as a copy of #1018394