(VAR X Y M N X1 X2 X3)
(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
)