  There is a natural bijection between permutations obtainable using a stack
(those avoiding the pattern 312) and permutations obtainable using a queue
(those avoiding 321). This bijection is equivalent to one described by Simion
and Schmidt in 1985. We argue that this bijection might well have been found
back in 1968 by readers of volume 1 of Knuth's *The Art of Computer
Programming*, if Knuth had not assigned difficulty ratings to his exercises.
