TRS:
 {          a__zeros() -> cons(0(), zeros()),
  a__tail(cons(X, XS)) -> mark(XS),
         mark(zeros()) -> a__zeros(),
         mark(tail(X)) -> a__tail(mark(X)),
    mark(cons(X1, X2)) -> cons(mark(X1), X2),
             mark(0()) -> 0(),
            a__zeros() -> zeros(),
            a__tail(X) -> tail(X)}
 Fail