The relevant paper, "Skip Lists: A Probabilistic
Alternative to Balanced Trees": <a href="http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.117.6383&rep=rep1&type=pdf" rel="nofollow">http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.117...</a>