DATA_SECTION
  init_int nobs
  init_vector Y(1,nobs)
  init_vector x(1,nobs)
PARAMETER_SECTION
  init_number a   
  init_number b   
  vector pred_Y(1,nobs)
  objective_function_value f
PROCEDURE_SECTION
  cout << "sd_phase = " << sd_phase << "; sd_phase() = " << sd_phase()
       << ", initial_params::sd_phase = " << initial_params::sd_phase
       << endl;
  pred_Y=a*x+b;
  f=(norm2(pred_Y-Y)); 
  f=nobs/2.*log(f);    // make it a likelihood function so that
                       // covariance matrix is correct
