21.1 Let (X;)>i be i.i.d. with P(Xj = 1) = P(X, = 0) = ?. Let S, = Zizi Xi, and let Zn = 257 -n. (Zn is the excess of heads over tails in n tosses, if X; = 1 when heads and X; = 0 when tails on the gth toss.) Show that lim P Un