main :: [sys_message]
main = [Stdout (lay (map test tests))]
       where test (n,x,y) = "F" ++ show n ++ "(" ++ show x ++ ","
                                ++ show y ++ ") = " ++ show (f n x y)
             tests = [(0,0,0), (1,1,1), (1,3,3),
                      (2,1,1), (2,2,1), (3,1,1)]

f :: num->num->num->num
f 0     x y     = x+y
f (n+1) x 0     = x, if n >= 0
f (n+1) x (y+1) = f n k (k + y + 1), if n >= 0 where k = f (n+1) x y