default Seq<T> concat(T other) {
        return concat(Seq.of(other));
    }
Concatenate two streams.
<p>
<pre><code>
// (1, 2, 3, 4)
Seq.of(1, 2, 3).concat(4)
</code></pre>

@see #concat(Stream[])
jOOL/src/main/java/org/jooq/lambda/Seq.java