code: public static <C extends Comparable<? super C>>
  	Collector<C, ?, MinMax<C>> toMinMax() {
  		return toMinMax(Comparator.naturalOrder());
  	}
docs: Return a {@code Collector} which calculates the minimum and maximum value.
  The <i>reducing</i> objects must be comparable.
  
  <pre>{@code
  final Stream<SomeObject> stream = ...
  final MinMax<SomeObject> moments = stream
  .collect(doubleMoments.toMinMax(comparator));
  }</pre>
  
  @param <C> the type of the input elements
  @return a {@code Collector} implementing the min-max reduction
  @throws java.lang.NullPointerException if the given {@code mapper} is
  {@code null}