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