},
        EnumMapAccumulator::combine,
        EnumMapAccumulator::toImmutableMap);
  }

  private static final class EnumMapAccumulator<K extends Enum<K>, V> {
    private final BinaryOperator<V> mergeFunction;
    private @Nullable EnumMap<K, V> map = null;

    EnumMapAccumulator(BinaryOperator<V> mergeFunction) {
      this.mergeFunction = mergeFunction;
    }