code: public NavigableMap<K, V> toNavigableMap() {
          NavigableMap<K, V> map = isParallel() ? new ConcurrentSkipListMap<>() : new TreeMap<>();
          forEach(toMapConsumer(map));
          return map;
      }
docs: Returns a {@link NavigableMap} containing the elements of this stream.
  There are no guarantees on the type or serializability of the
  {@code NavigableMap} returned; if more control over the returned
  {@code Map} is required, use {@link #toCustomMap(Supplier)}.
  
  <p>
  This is a <a href="package-summary.html#StreamOps">terminal</a>
  operation.
  
  <p>
  Returned {@code NavigableMap} is guaranteed to be modifiable.
  
  <p>
  For parallel stream the concurrent {@code NavigableMap} is created.
  
  @return a {@code NavigableMap} containing the elements of this stream
  @see Collectors#toMap(Function, Function)
  @see Collectors#toConcurrentMap(Function, Function)
  @throws IllegalStateException if this stream contains duplicate keys
  (according to {@link Object#equals(Object)})
  @since 0.6.5