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