factTU :: Int -> Int 
u :: Int -> Int -> Int -> Int
factTU(x) -> u(x, 1, 1)
u(x, i, a) -> a | i>x 
u(x, i, a) -> u(x, i+1, i*a) | i<=x

factRU :: Int -> Int 
U :: Int -> Int -> Int 
factRU(x) -> U(1, x)
U(i, x) -> x | i>x-1
U(i, x) -> i*U(i+1, x) | i<=x-1

tailup :: (Int -> Int -> Int) -> Int -> Int -> Int -> Int
tailup(F, i, y, a) -> a                               | i > y
tailup(F, i, y, a) -> tailup(F, i + 1, y, F(i, a))    | i ≤ y

recup :: (Int -> Int -> Int) -> Int -> Int -> Int -> Int
recup(F, i, y, a) -> a                                | i > y
recup(F, i, y, a) -> F(recup(F, i + 1, y, a), i)      | i ≤ y

helper :: Int -> Int -> Int
helper(x, y) -> y*x