def METHOD_NAME(ds, ad, cfl, iteration):
    if maxwell_solver == 'yee':
        dt = 1./scc.c * 1./np.sqrt((1./ad['dx'][-1]**2 + 1./ad['dy'][-1]**2 + 1./ad['dz'][-1]**2))
    elif maxwell_solver == 'ckc':
        dt = cfl * min( [ ad['dx'][-1], ad['dy'][-1], ad['dz'][-1] ] )  / scc.c
    z_front = dt * float(iteration) * scc.c + 7.5e-6*yt.units.meter
    z_shift = z_front-ds.domain_right_edge[2]
    return z_shift