dy = F_ADD(f, dy, a->x, dx); /* %$ x + x' $% */
dy = F_MUL(f, dy, dy, lambda); /* %$ (x + x') \lambda$% */
dy = F_ADD(f, dy, dy, a->y); /* %$ (x + x') \lambda + y$% */
dy = F_ADD(f, dy, a->x, dx); /* %$ x + x' $% */
dy = F_MUL(f, dy, dy, lambda); /* %$ (x + x') \lambda$% */
dy = F_ADD(f, dy, dy, a->y); /* %$ (x + x') \lambda + y$% */