YES
Input TRS:
    1: a() -> a() | f(a()) --> a()
    2: f(x) -> a() | x --> b()
Infeasibility test:
    f(a()) --> a()
Co-Order(NegReal,≥,Sum) ... succeeded.
     a()	weight: (- (/ 1 2))
     b()	weight: 0
     f(x1)	weight: (- (/ 1 2)) + x1
    #(x1)	weight: x1