TRS:
 {    a__f(a(), X, X) -> a__f(X, a__b(), b()),
               a__b() -> a(),
  mark(f(X1, X2, X3)) -> a__f(X1, mark(X2), X3),
            mark(b()) -> a__b(),
            mark(a()) -> a(),
     a__f(X1, X2, X3) -> f(X1, X2, X3),
               a__b() -> b()}
 Fail