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