(VAR N X XS Y )
(RULES 
        fib(N) -> sel(N, fib1(s(0), s(0)))
        fib1(X, Y) -> cons(X, fib1(Y, add(X, Y)))
        add(0, X) -> X
        add(s(X), Y) -> s(add(X, Y))
        sel(0, cons(X, XS)) -> X
        sel(s(N), cons(X, XS)) -> sel(N, XS)
        
)