(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)
active(cons(X1,X2)) -> cons(active(X1),X2)
active(incr(X)) -> incr(active(X))
active(s(X)) -> s(active(X))
active(head(X)) -> head(active(X))
active(tail(X)) -> tail(active(X))
cons(mark(X1),X2) -> mark(cons(X1,X2))
incr(mark(X)) -> mark(incr(X))
s(mark(X)) -> mark(s(X))
head(mark(X)) -> mark(head(X))
tail(mark(X)) -> mark(tail(X))
proper(nats) -> ok(nats)
proper(cons(X1,X2)) -> cons(proper(X1),proper(X2))
proper(0) -> ok(0)
proper(incr(X)) -> incr(proper(X))
proper(pairs) -> ok(pairs)
proper(odds) -> ok(odds)
proper(s(X)) -> s(proper(X))
proper(head(X)) -> head(proper(X))
proper(tail(X)) -> tail(proper(X))
cons(ok(X1),ok(X2)) -> ok(cons(X1,X2))
incr(ok(X)) -> ok(incr(X))
s(ok(X)) -> ok(s(X))
head(ok(X)) -> ok(head(X))
tail(ok(X)) -> ok(tail(X))
top(mark(X)) -> top(proper(X))
top(ok(X)) -> top(active(X))
)