running with 13 ingredients
factorial :: Int -> Int
-- testing 6 combinations of argument values
-- pruning with 67/101 rules
-- 3 candidates of size 1
-- 1 candidates of size 2
-- 4 candidates of size 3
-- 8 candidates of size 4
-- 28 candidates of size 5
-- 109 candidates of size 6
-- tested 146 candidates
factorial x  =  foldr (*) 1 [1..x]