(VAR X Y Z) (RULES f(X) -> cons(X,n__f(n__g(X))) g(0) -> s(0) g(s(X)) -> s(s(g(X))) sel(0,cons(X,Y)) -> X sel(s(X),cons(Y,Z)) -> sel(X,activate(Z)) f(X) -> n__f(X) g(X) -> n__g(X) activate(n__f(X)) -> f(activate(X)) activate(n__g(X)) -> g(activate(X)) activate(X) -> X )