default <K, A, D> Map<K, D> groupBy(Function<? super T, ? extends K> classifier, Collector<? super T, A, D> downstream) {
        return collect(Collectors.groupingBy(classifier, downstream));
    }
Shortcut for calling {@link Stream#collect(Collector)} with a
{@link Collectors#groupingBy(Function, Collector)} collector.
jOOL-java-8/src/main/java/org/jooq/lambda/Seq.java