Roland Carl Backhouse Maarten M. Fokkinga The associativity of equivalence and the Towers of Hanoi problem. 71-76 2001 77 Inf. Process. Lett. 2-4 db/journals/ipl/ipl77.html#BackhouseF01 http://dx.doi.org/10.1016/S0020-0190(00)00205-2