Because when all is said and done, you will have tried some finite number of things before you succeeded, Every time you made a mistake you eliminated one of those things and are one step closer.

The algorithm USES multi-time steps to replace single time step in traditional non-uniform finite-difference time-domain (NU-FDTD) method to enhance the computing speed.

For every time step, the two dimensional Laplace's equation is solved by a finite-element method, so the potential distribution over the foil is given.