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

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