(VAR x xs ys )
(RULES 
        f(true,xs) -> f(isList(xs),append(cons(a,nil),xs))
        isList(nil) -> true
        isList(cons(x,xs)) -> isList(xs)
        append(xs,ys) -> appendAkk(reverse(xs),ys)
        appendAkk(nil,ys) -> ys
        appendAkk(cons(x,xs),ys) -> appendAkk(xs,cons(x,ys))
        reverse(nil) -> nil
        reverse(cons(x,xs)) -> append(reverse(xs),cons(x,nil))
        
)