code: public static <K, V> Lens.Simple<Map<K, V>, Maybe<V>> valueAt(K k) {
          return adapt(valueAt(HashMap::new, k));
      }
docs: A lens that focuses on a value at a key in a map, as a {@link Maybe}.
  
  @param <K> the key type
  @param <V> the value type
  @param k   the key to focus on
  @return a lens that focuses on the value at key, as a {@link Maybe}