(VAR N X X1 X2 XS Y )
(RULES 
        a__fib(N) -> a__sel(mark(N), a__fib1(s(0), s(0)))
        a__fib1(X, Y) -> cons(mark(X), fib1(Y, add(X, Y)))
        a__add(0, X) -> mark(X)
        a__add(s(X), Y) -> s(a__add(mark(X), mark(Y)))
        a__sel(0, cons(X, XS)) -> mark(X)
        a__sel(s(N), cons(X, XS)) -> a__sel(mark(N), mark(XS))
        mark(fib(X)) -> a__fib(mark(X))
        mark(sel(X1, X2)) -> a__sel(mark(X1), mark(X2))
        mark(fib1(X1, X2)) -> a__fib1(mark(X1), mark(X2))
        mark(add(X1, X2)) -> a__add(mark(X1), mark(X2))
        mark(s(X)) -> s(mark(X))
        mark(0) -> 0
        mark(cons(X1, X2)) -> cons(mark(X1), X2)
        a__fib(X) -> fib(X)
        a__sel(X1, X2) -> sel(X1, X2)
        a__fib1(X1, X2) -> fib1(X1, X2)
        a__add(X1, X2) -> add(X1, X2)
        
)