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