@Override
  public @NotNull FMap<K, V> plus(@NotNull K key, @NotNull V value) {
    if (myKey.equals(key)) {
      if (myValue.equals(value)) {
        return this;
      }
      else {
        return new OneKeyFMap<>(key, value);
      }
    }
    else {
      return new TwoKeysFMap<>(myKey, myValue, key, value);
    }
  }
@Override
  public @NotNull FMap<K, V> plus(@NotNull K key, @NotNull V value)