Input TRS:
    1: a__f(a(),X,X) -> a__f(X,a__b(),b())
    2: a__b() -> a()
    3: mark(f(X1,X2,X3)) -> a__f(X1,mark(X2),X3)
    4: mark(b()) -> a__b()
    5: mark(a()) -> a()
    6: a__f(X1,X2,X3) -> f(X1,X2,X3)
    7: a__b() -> b()
Number of Rules: 7
Direct POLO(mSum) ... failed.