public Optional<Map.Entry<K, V>> minByValue(Comparator<V> comparator) {
        return inner.min(byValueOnly(requireNonNull(comparator)));
    }
Returns the minimum element of this stream according to the provided
{@code Comparator} applied to the value-components of this stream. This 
is a special case of a reduction.
<p>
This is a terminal operation.

@param comparator  a non-interfering, stateless {@code Comparator} to 
                   compare values of this stream
@return            an {@code Optional} describing the minimum element of 
                   this stream, or an empty {@code Optional} if the 
                   stream is empty

@throws NullPointerException if the minimum element is null or if any of the
        provided parameters are {@code null}
common-parent/mapstream/src/main/java/com/speedment/common/mapstream/MapStream.java