n your own predicate function.</p>
<p>You can either choose a custom message or just let the built-in messages be printed:</p>
<div class="sourceCode" id="cb27"><pre class="sourceCode r"><code class="sourceCode r"><a class="sourceLine" id="cb27-1" title="1">x &lt;-<span class="st"> </span><span class="dv">12</span></a>
<a class="sourceLine" id="cb27-2" title="2"><span class="co"># Stop if .x is numeric</span></a>
<a class="sourceLine" id="cb27-3" title="3"><span class="kw"><a href="reference/messagehandler.