is example shows how an array of <code>[Int]</code> instances can be joined, using
another <code>[Int]</code> instance as the separator:</p>

<pre><code class="language-swift">let nestedNumbers = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
let joined = nestedNumbers.joined(separator: [-1, -2])
print(Array(joined))
// Prints &quot;[1, 2, 3, -1, -2, 4, 5, 6, -1, -2, 7, 8, 9]&quot;</code></pre>

<p><strong><code>separator</code>:</strong>  A sequence to insert between each of this
  sequence&#39;s elements.
<strong>Retu