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