docs: public static <K, V> Lens.Simple<Map<K, V>, Set<K>> keys() {
          return simpleLens(m -> new HashSet<>(m.keySet()), (m, ks) -> {
              HashSet<K> ksCopy = new HashSet<>(ks);
              Map<K, V> updated = new HashMap<>(m);
              Set<K> keys = updated.keySet();
              keys.retainAll(ksCopy);
              ksCopy.removeAll(keys);
              ksCopy.forEach(k -> updated.put(k, null));
              return updated;
          });
      }