docs: public static <A, SetA extends Set<A>> Lens.Simple<SetA, Boolean> contains(
              Function<? super SetA, ? extends SetA> copyFn, A a) {
          return simpleLens(setA -> setA.contains(a),
                            (setA, include) -> {
                                SetA copy = copyFn.apply(setA);
                                if (include) copy.add(a);
                                else copy.remove(a);
                                return copy;
                            });
      }