(set! *unchecked-math* true)
(def fib (memoize (fn [x] (if (< x 2) x
                               (+' (fib (dec x)
                               (fib (- x 2)))))))
(time (fib 1000))
"Elapsed time: 12.914451 msecs"
;43466557686937456435688527675040625802564660517371780402481729089536555417949051890403879840079255169295922593080322634775209689623239873322471161642996440906533187938298969649928516003704476137795166849228875N
(time (fib 1000))
"Elapsed time: 0.155083 msecs"
;43466557686937456435688527675040625802564660517371780402481729089536555417949051890403879840079255169295922593080322634775209689623239873322471161642996440906533187938298969649928516003704476137795166849228875N