Input TRS:
    1: active(f(f(a()))) -> mark(f(g(f(a()))))
    2: mark(f(X)) -> active(f(mark(X)))
    3: mark(a()) -> active(a())
    4: mark(g(X)) -> active(g(X))
    5: f(mark(X)) -> f(X)
    6: f(active(X)) -> f(X)
    7: g(mark(X)) -> g(X)
    8: g(active(X)) -> g(X)
Number of Rules: 8
Direct POLO(Sum) ... failed.