running with 13 primitives
factorial :: Int -> Int
-- testing 6 combinations of argument values
-- pruning with 67/100 rules
-- looking through 3 candidates of size 1
-- looking through 7 candidates of size 2
-- looking through 22 candidates of size 3
-- looking through 62 candidates of size 4
-- looking through 175 candidates of size 5
-- looking through 542 candidates of size 6
factorial 0  =  1
factorial x  =  x * factorial (dec x)