ine">\((-\pi, \pi)\)</span> radians
can be declared as a two-dimensional unit vector then transformed to
an angle.</p>
<pre><code>parameters {
  unit_vector[2] xy;

transformed parameters {
  real&lt;lower = -pi(), upper = pi&gt; theta = atan2(xy[2], xy[1]);</code></pre>
<p>If the distribution of <span class="math inline">\((x, y)\)</span> is uniform over a circle, then the
distribution of <span class="math inline">\(\arctan \frac{y}{x}\)</span> is uniform over <span class="math inline">\((-\pi, pi]\)</span