default Seq<T> append(Optional<? extends T> other) {
        return concat(other);
    }
Concatenate an optional value.
<p>
<pre><code>
// (1, 2, 3, 4)
Seq.of(1, 2, 3).append(Optional.of(4))

// (1, 2, 3)
Seq.of(1, 2, 3).append(Optional.empty())
</code></pre>
jOOL-java-8/src/main/java/org/jooq/lambda/Seq.java