gcd :: Int -> Int -> Int
-- testing 11 combinations of argument values
-- pruning with 0/0 rules
-- looking through 3 candidates of size 1
-- looking through 8 candidates of size 2
-- looking through 20 candidates of size 3
-- looking through 60 candidates of size 4
-- looking through 150 candidates of size 5
-- looking through 472 candidates of size 6
-- tested 246 candidates
gcd x 0  =  x
gcd x y  =  gcd y (x `mod` y)