gcd :: Int -> Int -> Int
-- testing 11 combinations of argument values
-- looking through 3 candidates of size 1
-- looking through 0 candidates of size 2
-- looking through 9 candidates of size 3
-- looking through 0 candidates of size 4
-- looking through 54 candidates of size 5
-- looking through 0 candidates of size 6
-- looking through 405 candidates of size 7
-- looking through 90 candidates of size 8
-- looking through 3402 candidates of size 9
-- looking through 2016 candidates of size 10
gcd a b  =  if a == 0 then b else gcd (b `mod` a) a