default Seq<T> retainAll(Seq<? extends T> other) {
        Set<? extends T> set = other.toSet(HashSet::new);
        return set.isEmpty() ? empty() : filter(t -> set.contains(t)).onClose(other::close);
    }
Return a new stream where only occurrences of the arguments are retained.
<p>
<pre><code>
// 2, 3, 2
Seq.of(1, 2, 3, 2, 4).retainAll(2, 3)
</code></pre>
jOOL-java-8/src/main/java/org/jooq/lambda/Seq.java