topifnot</a></span><span class="op">(</span><span class="va">x</span> <span class="op">&gt;</span> <span class="fl">0</span><span class="op">)</span><span class="op">)</span>
  <span class="va">x</span>
<span class="op">}</span>
<span class="va">sess</span><span class="op">$</span><span class="fu">run</span><span class="op">(</span><span class="fu">stop_if_negative4</span><span class="op">(</span><span class="va">x</span><span class="op">)</span><span class="op">)</span>
<span class="co"># [&lt;#&gt; Error 