YES Problem: 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() Proof: WPO Processor: algebra: Max weight function: w0 = 0 w(zprimes) = w(a__zprimes) = 5 w(nats) = w(a__nats) = w(sieve) = w(a__sieve) = w(mark) = w(s) = w( filter) = w(a__filter) = w(0) = w(cons) = 0 status function: st(filter) = [2, 0, 1] st(zprimes) = st(a__zprimes) = [] st(nats) = st(a__nats) = st(sieve) = st(a__sieve) = st(mark) = st( s) = [0] st(a__filter) = [0, 1, 2] st(0) = [] st(cons) = [0, 1] subterm penalty function: sp(nats, 0) = sp(a__nats, 0) = 4 sp(cons, 0) = 2 sp(sieve, 0) = sp(a__sieve, 0) = sp(mark, 0) = sp(s, 0) = sp(filter, 2) = sp( filter, 1) = sp(filter, 0) = sp(a__filter, 2) = sp(a__filter, 1) = sp( a__filter, 0) = sp(cons, 1) = 0 precedence: mark > a__nats ~ a__sieve > a__zprimes ~ a__filter > zprimes ~ nats ~ sieve ~ s ~ filter ~ 0 ~ cons problem: Qed