factorial :: Int -> Int
-- testing 4 combinations of argument values
-- pruning with 4/8 rules
-- looking through 2 candidates of size 1
-- looking through 1 candidates of size 2
-- looking through 1 candidates of size 3
-- looking through 1 candidates of size 4
-- looking through 1 candidates of size 5
-- looking through 9 candidates of size 6
factorial x  =  foldr (*) 1 [1..x]

factorial :: Int -> Int
-- testing 4 combinations of argument values
-- pruning with 22/42 rules
-- looking through 3 candidates of size 1
-- looking through 6 candidates of size 2
-- looking through 16 candidates of size 3
-- looking through 39 candidates of size 4
-- looking through 78 candidates of size 5
-- looking through 166 candidates of size 6
factorial 0  =  1
factorial x  =  x * factorial (dec x)