A closure that accepts an element of this
  sequence as its argument and returns an optional value.
<strong>Returns:</strong> An array of the non-<code>nil</code> results of calling <code>transform</code>
  with each element of the sequence.</p>

<p><strong>Complexity:</strong> O(<em>m</em> + <em>n</em>), where <em>m</em> is the length of this sequence
  and <em>n</em> is the length of the result.</p>

    <h4>Declaration</h4>    
    <code class="language-swift">func flatMap&lt;ElementOfResult&gt;(_ transf