an>char <span class="arrow">&#45;&gt;</span></span> <span>int option</span></span></code></div></div><div class="odoc-spec"><div class="spec value anchored" id="val-index_opt"><a href="#val-index_opt" class="anchor"></a><code><span><span class="keyword">val</span> index_opt : <span>string <span class="arrow">&#45;&gt;</span></span> <span>char <span class="arrow">&#45;&gt;</span></span> <span>int option</span></span></code></div></div><div class="odoc-spec"><div class="spec value anchored" id="val-rindex_opt