A true Scheme koan, due to Andrzej Filinski, with help from Franklyn Turbak and me: "What is the CPS transform of an infinite loop?" Specifically, rewrite (define ((yin f) a) ((yin f) (f a))) for f in CPS (curried, with continuation first, then argument). The result is (define ((yang f) a) ((f (yang f)) a)) Seem familiar? Those symbols on the cover of SICP stand for more than just eval and apply!