/* The other coefficients are changed so that the step is half the original step. This means that doing fd_fwd (f) n times with the input f results in the same f as doing fd_fwd (f) 2n times with the output f.  */
static void fd_down(double f[4])
/* The other coefficients are changed so that the step is half the original step. This means that doing fd_fwd (f) n times with the input f results in the same f as doing fd_fwd (f) 2n times with the output f.  */
static void fd_down(double f[4])
{
    f[3] *= 0.125;
    f[2] = f[2] * 0.25 - f[3];
    f[1] = (f[1] - f[2]) * 0.5;
}