# Better names due to Jim Weirich: http://vimeo.com/45140590
def (Y improver)
  ((fn(gen) gen.gen)
   (fn(gen)
     (fn(n)
       ((improver gen.gen) n))))

factorial <- (Y (fn(f)
                  (fn(n)
                    (if zero?.n
                      1
                      (n * (f n-1))))))

prn factorial.5