Input TRS:
    1: a__f(X,X) -> a__f(a(),b())
    2: a__b() -> a()
    3: mark(f(X1,X2)) -> a__f(mark(X1),X2)
    4: mark(b()) -> a__b()
    5: mark(a()) -> a()
    6: a__f(X1,X2) -> f(X1,X2)
    7: a__b() -> b()
Number of Rules: 7
Direct QWPOS(Sum) ... failed.