he log of the density of <code>y</code> given
parameter <code>theta</code> allows the use of the sampling statement is
defined as follows.</p>
<pre class="stan"><code>real foo_lpdf(real y, vector theta) { ... }</code></pre>
<p>Note that for function definitions, the comma is used rather than the
vertical bar.</p>
<p>For every custom <code>_lpdf</code> and <code>_lpmf</code> defined there is a corresponding
<code>_lupdf</code> and <code>_lupmf</code> defined automatically. The <code>_lupdf</code> and <code>_