">c</code> in <code class="code">s</code>, and the sub-string after the
   first occurrence of <code class="code">c</code> in <code class="code">s</code>.
   <code class="code"><span class="keyword">let</span>&nbsp;(before,&nbsp;after)&nbsp;=&nbsp;<span class="constructor">String</span>.cut_at&nbsp;s&nbsp;c&nbsp;<span class="keyword">in</span><br>
&nbsp;&nbsp;&nbsp;&nbsp;before&nbsp;^&nbsp;<span class="constructor">String</span>.make&nbsp;1&nbsp;c&nbsp;^&nbsp;after</code> is the identity if <code class="cod