n Stan by names ending in
<code>_lpdf</code> for density functions and <code>_lpmf</code> for mass
functions; in both cases, they must have <code>real</code> return types.</p>
<p>Suppose a model uses several standard normal distributions, for which
there is not a specific overloaded density nor defaults in Stan. So
rather than writing out the location of 0 and scale of 1 for all of
them, a new density function may be defined and reused.</p>
<pre><code>functions {
  real unit_normal_lpdf(real y) {
    return