(VAR M N X X1 X2 X3 Y )
(RULES 
        active(filter(cons(X, Y), 0, M)) -> mark(cons(0, filter(Y, M, M)))
        active(filter(cons(X, Y), s(N), M)) -> mark(cons(X, filter(Y, N, M)))
        active(sieve(cons(0, Y))) -> mark(cons(0, sieve(Y)))
        active(sieve(cons(s(N), Y))) -> mark(cons(s(N), sieve(filter(Y, N, N))))
        active(nats(N)) -> mark(cons(N, nats(s(N))))
        active(zprimes) -> mark(sieve(nats(s(s(0)))))
        mark(filter(X1, X2, X3)) -> active(filter(mark(X1), mark(X2), mark(X3)))
        mark(cons(X1, X2)) -> active(cons(mark(X1), X2))
        mark(0) -> active(0)
        mark(s(X)) -> active(s(mark(X)))
        mark(sieve(X)) -> active(sieve(mark(X)))
        mark(nats(X)) -> active(nats(mark(X)))
        mark(zprimes) -> active(zprimes)
        filter(mark(X1), X2, X3) -> filter(X1, X2, X3)
        filter(X1, mark(X2), X3) -> filter(X1, X2, X3)
        filter(X1, X2, mark(X3)) -> filter(X1, X2, X3)
        filter(active(X1), X2, X3) -> filter(X1, X2, X3)
        filter(X1, active(X2), X3) -> filter(X1, X2, X3)
        filter(X1, X2, active(X3)) -> filter(X1, X2, X3)
        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)
        s(mark(X)) -> s(X)
        s(active(X)) -> s(X)
        sieve(mark(X)) -> sieve(X)
        sieve(active(X)) -> sieve(X)
        nats(mark(X)) -> nats(X)
        nats(active(X)) -> nats(X)
        
)