(VAR X X1 X2 )
(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)
        
)