(VAR N XS YS X 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
)