fcn Subroutine

   pure subroutine fcn(beta, xplusd, ifixb, ifixx, ideval, f, fjacb, fjacd, istop)
   !! User-supplied subroutine for evaluating the model.

      integer, intent(in) :: ideval, ifixb(:), ifixx(:, :)
      real(kind=wp), intent(in) :: beta(:), xplusd(:, :)
      real(kind=wp), intent(out) :: f(:, :), fjacb(:, :, :), fjacd(:, :, :)
      integer, intent(out) :: istop

      ! Local variables
      integer :: i

      ! Check for unacceptable values for this problem
      if (beta(1) > zero) then
         istop = 1
         return
      else
         istop = 0
      end if

      ! Compute predicted values
      if (mod(ideval, 10) >= 1) then
         do i = 1, ubound(f, 2)
            f(:, i) = beta(3)*(xplusd(:, 1) - beta(1))**2 + &
                      2*beta(4)*(xplusd(:, 1) - beta(1))*(xplusd(:, 2) - beta(2)) + &
                      beta(5)*(xplusd(:, 2) - beta(2))**2 - one
         end do
      end if

   end subroutine fcn