Input TRS:
    1: a__U11(tt(),M,N) -> a__U12(tt(),M,N)
    2: a__U12(tt(),M,N) -> s(a__plus(mark(N),mark(M)))
    3: a__plus(N,0()) -> mark(N)
    4: a__plus(N,s(M)) -> a__U11(tt(),M,N)
    5: mark(U11(X1,X2,X3)) -> a__U11(mark(X1),X2,X3)
    6: mark(U12(X1,X2,X3)) -> a__U12(mark(X1),X2,X3)
    7: mark(plus(X1,X2)) -> a__plus(mark(X1),mark(X2))
    8: mark(tt()) -> tt()
    9: mark(s(X)) -> s(mark(X))
    10: mark(0()) -> 0()
    11: a__U11(X1,X2,X3) -> U11(X1,X2,X3)
    12: a__U12(X1,X2,X3) -> U12(X1,X2,X3)
    13: a__plus(X1,X2) -> plus(X1,X2)
Number of Rules: 13
Direct POLO(mSum) ... failed.