a</code> (technically, it will include <code>alpha</code> of size 0,
which means it won’t contain any values and won’t be included in any
output).</p>
<p>This technique is not just useful for containers. If the value of
<code>N</code> is set to 1, then the vector <code>alpha</code> will contain a
single element and thus <code>alpha[1]</code> behaves like an optional
scalar, the existence of which is controlled by <code>include_alpha</code>.</p>
<p>This coding pattern allows a single Stan program to defi