p>We can use <code>xc</code>, and define the <code>integrand</code> as:</p>
<div class="sourceCode" id="cb7"><pre class="sourceCode stan code-with-copy"><code class="sourceCode stan"><span id="cb7-1"><a href="#cb7-1" aria-hidden="true" tabindex="-1"></a><span class="dt">real</span> integrand(<span class="dt">real</span> x,</span>
<span id="cb7-2"><a href="#cb7-2" aria-hidden="true" tabindex="-1"></a>               <span class="dt">real</span> xc,</span>
<span id="cb7-3"><a href="#cb7-3" aria-hidden="true" t