> using <code class="code">cmp</code>. If the array is not sorted,
    the result is not specified (may raise Invalid_argument).
<p>

    Complexity: O(log n) where n is the length of the array
    (dichotomic search).<br>
<b>Since</b> 0.13<br>
<b>Raises</b> <code>Invalid_argument</code> if the array is found to be unsorted w.r.t <code class="code">cmp</code><br>
<b>Returns</b> - <code class="code">`At i</code> if <code class="code">cmp arr.(i) x = 0</code> (for some i)<ul>
<li><code class="code">`All_lower