Input TRS:
    1: a__f(f(a())) -> c(f(g(f(a()))))
    2: mark(f(X)) -> a__f(mark(X))
    3: mark(a()) -> a()
    4: mark(c(X)) -> c(X)
    5: mark(g(X)) -> g(mark(X))
    6: a__f(X) -> f(X)
Number of Rules: 6
Direct QKBOS(Sum) ... failed.