st occurrence of byte <code class="code">c</code>
    in <code class="code">s</code>.</p>
</div>
<ul class="info-attributes">
<li><b>Raises</b> <code>Not_found</code> if <code class="code">c</code> does not occur in <code class="code">s</code>.</li>
</ul>
</div>

<pre id="VALindex_opt">val index_opt : bytes -&gt; char -&gt; int option</pre><div class="info ">
<div class="info-desc">
<p><code class="code">index_opt&nbsp;s&nbsp;c</code> returns the index of the first occurrence of byte <code class="code">c</c