default <U> Seq<T> distinct(Function<? super T, ? extends U> keyExtractor) {
        final Map<U, String> seen = new ConcurrentHashMap<>();
        return filter(t -> seen.put(keyExtractor.apply(t), "") == null);
    }
Get a stream of distinct keys.
<p>
<pre><code>
// (1, 2, 3)
Seq.of(1, 1, 2, -2, 3).distinct(Math::abs)
</code></pre>
jOOL-java-8/src/main/java/org/jooq/lambda/Seq.java