@Override
  public @NotNull <T> KeyFMap plus(@NotNull Key<T> key, @NotNull T value) {
    if (myKey == key) {
      return value == myValue ? this : new OneElementFMap<>(key, value);
    }
    return new PairElementsFMap<>(myKey, myValue, key, value);
  }
@Override
  public @NotNull <T> KeyFMap plus(@NotNull Key<T> key, @NotNull T value)