this.mergeFunction = mergeFunction;
    }

    void put(K key, V value) {
      if (map == null) {
        map = new EnumMap<>(singletonMap(key, value));
      } else {
        map.merge(key, value, mergeFunction);
      }
    }

    EnumMapAccumulator<K, V> combine(EnumMapAccumulator<K, V> other) {
      if (this.map == null) {
        return other;
      } else if (other.map == null) {

      this.mergeFunction = mergeFunction;
    }

    void put(K key, V value) {
      if (map == null) {
        map = new EnumMap<>(singletonMap(key, value));
      } else {
        map.merge(key, value, mergeFunction);
      }
    }

    EnumMapAccumulator<K, V> combine(EnumMapAccumulator<K, V> other) {
      if (this.map == null) {
        return other;
      } else if (other.map == null) {