code> and <code class="code">l2</code> are sorted according to the
    comparison function <code class="code">cmp</code>, <code class="code">merge&nbsp;cmp&nbsp;l1&nbsp;l2</code> will return a
    sorted list containing all the elements of <code class="code">l1</code> and <code class="code">l2</code>.
    If several elements compare equal, the elements of <code class="code">l1</code> will be
    before the elements of <code class="code">l2</code>.
    Not tail-recursive (sum of the lengths of the arguments)