(VAR X XS X1 X2)
(RULES 
active(nats) -> mark(cons(0,incr(nats)))
active(pairs) -> mark(cons(0,incr(odds)))
active(odds) -> mark(incr(pairs))
active(incr(cons(X,XS))) -> mark(cons(s(X),incr(XS)))
active(head(cons(X,XS))) -> mark(X)
active(tail(cons(X,XS))) -> mark(XS)
mark(nats) -> active(nats)
mark(cons(X1,X2)) -> active(cons(mark(X1),X2))
mark(0) -> active(0)
mark(incr(X)) -> active(incr(mark(X)))
mark(pairs) -> active(pairs)
mark(odds) -> active(odds)
mark(s(X)) -> active(s(mark(X)))
mark(head(X)) -> active(head(mark(X)))
mark(tail(X)) -> active(tail(mark(X)))
cons(mark(X1),X2) -> cons(X1,X2)
cons(X1,mark(X2)) -> cons(X1,X2)
cons(active(X1),X2) -> cons(X1,X2)
cons(X1,active(X2)) -> cons(X1,X2)
incr(mark(X)) -> incr(X)
incr(active(X)) -> incr(X)
s(mark(X)) -> s(X)
s(active(X)) -> s(X)
head(mark(X)) -> head(X)
head(active(X)) -> head(X)
tail(mark(X)) -> tail(X)
tail(active(X)) -> tail(X)
)