static <A, B> L<A> lambdaMapLike uniquifyBy(IF1<A, B> f, Cl<A> l) {
  if (l == null) null;
  if (l(l) < 2) ret asList(l);
  new HashSet<B> set;
  new L<A> out;
  for (A a : l)
    if (set.add(f.get(a)))
      out.add(a);
  ret out;
}

static <A, B> L<A> lambdaMapLike uniquifyBy(Cl<A> l, IF1<A, B> f) {
  ret uniquifyBy(f, l);
}