aring Optional Variables</h3>
<p>A variable may be declared with a size that depends on a boolean
constant. For example, consider the definition of <code>alpha</code> in the
following program fragment.</p>
<pre class="stan"><code>data {
  int&lt;lower = 0, upper = 1&gt; include_alpha;
  ...
parameters {
  vector[include_alpha ? N : 0] alpha;</code></pre>
<p>If <code>include_alpha</code> is true, the model will include the vector
<code>alpha</code>; if the flag is false, the model will not include
<code>alph