элементарная ж вещь, но полезная

public static <X, Y> Map<Y, Set<X>> groupBy(Set<X> xs, Set<Y> ys, Function<X, Y> f) {
  Map<Y, Set<X>> result = new HashMap<Y, Set<X>>();
  for (Y y : ys) {
    result.put(y, new HashSet<X>());
  }

  for (X x : xs) {
    result.get(f.apply(x)).add(x);
  }

  return result;
}

меня немножко стремает что берём функцию а возвращаем мап, но...

Inner Class in Java: Unexpected Volatility

Just wrote

one more funny piece of funny code

public SetMorphism<Y, Set<Y>, Set<X>, Set<Set<X>>> revert() {
  Map<Y, Set<X>> reverse = Sets.groupBy(domain(), codomain(), asFunction());
  return Morphism(codomain(), Sets.powerset(domain()), forMap(reverse));
}

(as some Kokoito said, it's time to stop playing computer games)

going for vacation for a week,

will be back on 1/4; no computer, probably, so... have fun!