running with 13 primitives
factorial :: Int -> Int
-- testing 6 combinations of argument values
-- pruning with 67/101 rules
-- 3 candidates of size 1
-- 3 candidates of size 2
-- 6 candidates of size 3
-- 13 candidates of size 4
-- 40 candidates of size 5
-- 138 candidates of size 6
-- tested 69 candidates
factorial 0  =  1
factorial x  =  x * factorial (dec x)