public K reduceKeys(long parallelismThreshold,
                        BiFunction<? super K, ? super K, ? extends K> reducer) {
        if (reducer == null) throw new NullPointerException();
        return new ReduceKeysTask<K,V>
            (null, batchFor(parallelismThreshold), 0, 0, table,
             null, reducer).invoke();
    }
public K reduceKeys(long parallelismThreshold,
                        BiFunction<? super K, ? super K, ? extends K> reducer)