a strictly specified signature. For example, the
nonlinear system given above can be coded using the
following function in Stan (see the <a href="#functions-programming">user-defined functions
section</a> for more information on coding
user-defined functions).</p>
<pre><code>vector system(vector y,        // unknowns
              vector theta,    // parameters
              real[] x_r,      // data (real)
              int[] x_i) {     // data (integer)
  vector[2] z;
  z[1] = y[1] - theta[1];
  z[2] = y[1