docs: public static <T> Collector<T, ?, DoubleSummary>
  	toDoubleSummary(final ToDoubleFunction<? super T> mapper) {
  		requireNonNull(mapper);
  		return Collector.of(
  			DoubleSummaryStatistics::new,
  			(a, b) -> a.accept(mapper.applyAsDouble(b)),
  			(a, b) -> {a.combine(b); return a;},
  			DoubleSummary::of
  		);
  	}