"code">c</code> in <code class="code">s</code>.
   <code class="code"><span class="keyword">let</span> (before, after) = <span class="constructor">String</span>.cut_at s c <span class="keyword">in</span>
    before ^ <span class="constructor">String</span>.make 1 c ^ after</code> is the identity if <code class="code">s</code> contains <code class="code">c</code>.
<p>

   Raise <code class="code"><span class="constructor">Not_found</span></code> if the character does not appear in the string<br>
<b>Since</b>