(VAR x y )
(RULES 
        fib(0) -> 0
        fib(s(0)) -> s(0)
        fib(s(s(0))) -> s(0)
        fib(s(s(x))) -> sp(g(x))
        g(0) -> pair(s(0), 0)
        g(s(0)) -> pair(s(0), s(0))
        g(s(x)) -> np(g(x))
        sp(pair(x, y)) -> +(x, y)
        np(pair(x, y)) -> pair(+(x, y), x)
        +(x, 0) -> x
        +(x, s(y)) -> s(+(x, y))
        
)