Right, similarly at time T minus one we have  Y, Y T minus one equals alpha plus beta  X T minus one  plus lander plus lander X T minus two, plus lander squared  X T minus three plus  X T minus four  infinity again  we call that B and that's just the same equation but we just  erm shifted the subscripts and right, this explains why in T, this ex explains why in T minus one  right, all the subscripts have just been  just been changed.