(VAR M N X X1 X2 X3 Y )
(RULES 
        filter(cons(X, Y), 0, M) -> cons(0, n__filter(activate(Y), M, M))
        filter(cons(X, Y), s(N), M) -> cons(X, n__filter(activate(Y), N, M))
        sieve(cons(0, Y)) -> cons(0, n__sieve(activate(Y)))
        sieve(cons(s(N), Y)) -> cons(s(N), n__sieve(filter(activate(Y), N, N)))
        nats(N) -> cons(N, n__nats(s(N)))
        zprimes -> sieve(nats(s(s(0))))
        filter(X1, X2, X3) -> n__filter(X1, X2, X3)
        sieve(X) -> n__sieve(X)
        nats(X) -> n__nats(X)
        activate(n__filter(X1, X2, X3)) -> filter(X1, X2, X3)
        activate(n__sieve(X)) -> sieve(X)
        activate(n__nats(X)) -> nats(X)
        activate(X) -> X
        
)