// A: object type
// B: hash type (e.g. Long)
// The hasher is just for speeding up the process.
// Every object is also checked using eq() among those with the same hash.
static <A, B> L<A> lambdaMapLike uniquifyUsingHasher(IF1<A, B> hasher, Iterable<A> l) {
  ret new UniquifyUsingHasher(hasher).addAll(l)!;
}