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