@Override
  public @NotNull <V> KeyFMap plus(@NotNull Key<V> key, @NotNull V value) {
    UnmodifiableHashMap<Key<?>, Object> newMap = myMap.with(key, value);
    return newMap == myMap ? this : new DebugFMap(newMap);
  }
@Override
  public @NotNull <V> KeyFMap plus(@NotNull Key<V> key, @NotNull V value)