sp;l</code> returns a pair of lists <code class="code">(l1,&nbsp;l2)</code> such that,
    for each element <code class="code">x</code> of the input list <code class="code">l</code>:</p>
<ul>
<li>if <code class="code">f&nbsp;x</code> is <code class="code">Left&nbsp;y1</code>, then <code class="code">y1</code> is in <code class="code">l1</code>, and</li>
<li>if <code class="code">f&nbsp;x</code> is <code class="code">Right&nbsp;y2</code>, then <code class="code">y2</code> is in <code class="code">l2</code>.<