(VAR N X XS YS ZS )
(RULES 
        natsFrom(N) -> cons(N, n__natsFrom(s(N)))
        fst(pair(XS, YS)) -> XS
        snd(pair(XS, YS)) -> YS
        splitAt(0, XS) -> pair(nil, XS)
        splitAt(s(N), cons(X, XS)) -> u(splitAt(N, activate(XS)), N, X, activate(XS))
        u(pair(YS, ZS), N, X, XS) -> pair(cons(activate(X), YS), ZS)
        head(cons(N, XS)) -> N
        tail(cons(N, XS)) -> activate(XS)
        sel(N, XS) -> head(afterNth(N, XS))
        take(N, XS) -> fst(splitAt(N, XS))
        afterNth(N, XS) -> snd(splitAt(N, XS))
        natsFrom(X) -> n__natsFrom(X)
        activate(n__natsFrom(X)) -> natsFrom(X)
        activate(X) -> X
        
)