default <K, A, D> Seq<Tuple2<K, D>> grouped(Function<? super T, ? extends K> classifier, Collector<? super T, A, D> downstream) {
        return grouped(this, classifier, downstream);
    }
Classify this stream's elements according to a given classifier function
and collect each class's elements using a collector.
<p>
<pre><code>
// Seq(tuple(1, 9), tuple(0, 12))
Seq.of(1, 2, 3, 4, 5, 6).grouped(i -> i % 2, Collectors.summingInt(i -> i))
// Seq(tuple(true, 9), tuple(false, 12))
Seq.of(1, 2, 3, 4, 5, 6).grouped(i -> i % 2 != 0, Collectors.summingInt(i -> i))
</code></pre> This is a non-terminal analog of
{@link #groupBy(Function, Collector)})

@see #groupBy(Function, Collector)
jOOL/src/main/java/org/jooq/lambda/Seq.java