s="type-var">'a</span> <a href="#type-t">t</a></span></span></code></div><div class="spec-doc"><p><code>alt p q</code> is <code>p &lt;|&gt; q</code>. See <code>Reparse</code>.Infix.((&lt;|&gt;))</p></div></div><div class="odoc-spec"><div class="spec value" id="val-optional" class="anchored"><a href="#val-optional" class="anchor"></a><code><span><span class="keyword">val</span> optional : <span><span><span class="type-var">'a</span> <a href="#type-t">t</a></span> <span class="arrow">&#45;&gt;</span></span> <