If the true solution to Eq. 1 is known, you can supply it in function true(x,y) and the x and y derivatives in function truex(x,y) and function truey(x,y), respectively. These function subroutines should be in the file user.f. With these functions defined, you can measure the error between the true and computed solutions. If you do not know the true solution, you should use true = 0.0, in which case the printed and graphical values of the ``error'' are the norm of the computed solution.