-- Peano natural numbers and arithmetic in rFun (reversible functional language)
data Nat = Z | S Nat

plus :: Nat -> Nat <-> Nat
plus Z     x = x
plus (S y) x =
  let x' = plus y x
  in  (S x')

minus :: Nat -> Nat <-> Nat
minus y x = plus! y x

-- The classical Fibonacci function (embedded to result in a pair)
-- fib n gives the (n+1)-th and n-th Fibonacci numbers
fib :: Nat <-> (Nat, Nat)
fib Z    = ((S Z),Z)
fib (S m) =
  let (x,y) = fib m
      y' = plus x y
  in (y', x)