(VAR X1 X2 X) (RULES active(f(0)) -> mark(cons(0,f(s(0)))) active(f(s(0))) -> mark(f(p(s(0)))) active(p(s(0))) -> mark(0) mark(f(X)) -> active(f(mark(X))) mark(0) -> active(0) mark(cons(X1,X2)) -> active(cons(mark(X1),X2)) mark(s(X)) -> active(s(mark(X))) mark(p(X)) -> active(p(mark(X))) f(mark(X)) -> f(X) f(active(X)) -> f(X) 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) p(mark(X)) -> p(X) p(active(X)) -> p(X) )