An urn contains black and red balls. Let Zn be the proportion of black balls at time n and 0 ≤ L < U ≤ 1 random barriers. At each time n, a ball bn is drawn. If bn is black and Zn−1 < U , then bn is replaced together with a random number Bn of black balls. If bn is red and Zn−1 > L, then bn is replaced together with a random number Rn of red balls. Otherwise, no additional balls are added, and ...