=+. Let D ,, be 1 or 0 according as X2 ,- 1 # X2 ,, or not, and let M, be the time of the kth 1-the smallest n such that Ej _, D, = k. Let Z, = X2M. In other words, look at successive nonoverlapping pairs (X2n - 1, X2,), discard accordant

(X2m-1 =X2) pairs, and keep the second element of discordant (X2m-1 *X2)