fac 0 = 1
fac n = n * fac (n-1)

fact n = product [1..n]

fakt n = foldl (*) 1 [1..n]

fib 0 = 0
fib 1 = 1
fib n = fib (n-1) + fib (n-2)

fib' n = fibs !! (n+1)
         where fibs = 0 : 1 : zipWith (+) fibs (tail fibs)

mtf n = n `mod` 5 == 0 || n `mod` 10 == 0