(VAR N X X1 X2 XS Y )
(RULES 
        fib(N) -> sel(N, fib1(s(0), s(0)))
        fib1(X, Y) -> cons(X, n__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, activate(XS))
        fib1(X1, X2) -> n__fib1(X1, X2)
        activate(n__fib1(X1, X2)) -> fib1(X1, X2)
        activate(X) -> X
        
)