dx = F_SUB(f, dx, dx, b->x); /* %$x' = \lambda^2 - x_0 - x_1$% */
dy = F_SUB(f, dy, b->x, dx); /* %$x_1 - x'$% */
dy = F_MUL(f, dy, lambda, dy); /* %$\lambda (x_1 - x')$% */
dx = F_SUB(f, dx, dx, b->x); /* %$x' = \lambda^2 - x_0 - x_1$% */
dy = F_SUB(f, dy, b->x, dx); /* %$x_1 - x'$% */
dy = F_MUL(f, dy, lambda, dy); /* %$\lambda (x_1 - x')$% */