s value <code>()</code> if and only if <code>p</code> fails to parse, otherwise the parse fails.</p><p>Examples</p><pre><code>module P = Reparse.String

;;
let p = P.(not_ (char 'a')) in
let v = P.parse_string p &quot;bbb&quot; in
v = ()</code></pre></div></div><div class="odoc-spec"><div class="spec value" id="val-is" class="anchored"><a href="#val-is" class="anchor"></a><code><span><span class="keyword">val</span> is : <span><span><span class="type-var">'a</span> <a href="#type-t">t</a></span> <span class