tri :: Int -> Int
-- testing 4 combinations of argument values
-- pruning with 11/33 rules
-- 2 candidates of size 1
-- 0 candidates of size 2
-- 5 candidates of size 3
-- 5 candidates of size 4
-- 10 candidates of size 5
-- 8 candidates of size 6
-- 41 candidates of size 7
-- tested 31 candidates
tri 1  =  1
tri x  =  x + tri (x - 1)