sum :: [Int] -> Int
-- testing 4 combinations of argument values
-- looking through 2 candidates of size 1
-- looking through 2 candidates of size 2
-- looking through 3 candidates of size 3
-- looking through 5 candidates of size 4
-- looking through 13 candidates of size 5
-- looking through 26 candidates of size 6
-- looking through 59 candidates of size 7
-- looking through 140 candidates of size 8
-- looking through 326 candidates of size 9
-- looking through 776 candidates of size 10
sum xs  =  if null xs then 0 else head xs + sum (tail xs)

sum :: [Int] -> Int
-- testing 6 combinations of argument values
-- looking through 2 candidates of size 1
-- looking through 2 candidates of size 2
-- looking through 3 candidates of size 3
-- looking through 5 candidates of size 4
-- looking through 13 candidates of size 5
-- looking through 26 candidates of size 6
-- looking through 59 candidates of size 7
-- looking through 140 candidates of size 8
-- looking through 326 candidates of size 9
-- looking through 776 candidates of size 10
sum xs  =  if null xs then 0 else head xs + sum (tail xs)

sum :: [Int] -> Int
-- testing 6 combinations of argument values
-- looking through 2 candidates of size 1
-- looking through 2 candidates of size 2
-- looking through 3 candidates of size 3
-- looking through 5 candidates of size 4
-- looking through 13 candidates of size 5
-- looking through 26 candidates of size 6
-- looking through 59 candidates of size 7
-- looking through 140 candidates of size 8
-- looking through 326 candidates of size 9
-- looking through 776 candidates of size 10
sum xs  =  if null xs then 0 else head xs + sum (tail xs)