sum :: [Int] -> Int
-- testing 4 combinations of argument values
-- pruning with 14/25 rules
-- looking through 2 candidates of size 1
-- looking through 3 candidates of size 2
-- looking through 3 candidates of size 3
-- looking through 7 candidates of size 4
-- looking through 11 candidates of size 5
-- tested 16 candidates
sum []  =  0
sum (x:xs)  =  x + sum xs

sum :: [Int] -> Int
-- pruning with 14/25 rules
-- looking through 2 candidates of size 1
-- looking through 5 candidates of size 2
-- looking through 5 candidates of size 3
-- looking through 12 candidates of size 4
-- looking through 18 candidates of size 5
-- tested 25 candidates
sum []  =  0
sum (x:xs)  =  x + sum xs