public ConcurrentNavigableMap<K, V> toConcurrentNavigableMap() {
        return inner.collect(Collectors.toConcurrentMap(
            Map.Entry::getKey,
            Map.Entry::getValue,
            throwingMerger(),
            ConcurrentSkipListMap::new
        ));
    }
Accumulates all key-value pairs of this {@code MapStream} into a 
{@code ConcurrentNavigableMap}.
<p>
If the mapped keys contains duplicates (according to
{@link Object#equals(Object)}), an {@code IllegalStateException} is
thrown when the collection operation is performed. If the mapped keys
may have duplicates, use {@link #toConcurrentNavigableMap(BinaryOperator)}
instead.
<p>
If the keys are not {@link Comparable}, a 
{@code java.lang.ClassCastException} may be thrown when the terminal 
operation is executed. It is therefore better to use 
{@link #toConcurrentNavigableMapByKey(Comparator)} since it allows you to specify 
a comparator to use when comparing the keys.

@return  a {@code ConcurrentNavigableMap} whose keys and values are 
         identical to the entries of this stream

@see #toMap(BinaryOperator)
@see #toConcurrentMap()
common-parent/mapstream/src/main/java/com/speedment/common/mapstream/MapStream.java