Input TRS:
    1: active(zeros()) -> mark(cons(0(),zeros()))
    2: active(tail(cons(X,XS))) -> mark(XS)
    3: active(cons(X1,X2)) -> cons(active(X1),X2)
    4: active(tail(X)) -> tail(active(X))
    5: cons(mark(X1),X2) -> mark(cons(X1,X2))
    6: tail(mark(X)) -> mark(tail(X))
    7: proper(zeros()) -> ok(zeros())
    8: proper(cons(X1,X2)) -> cons(proper(X1),proper(X2))
    9: proper(0()) -> ok(0())
    10: proper(tail(X)) -> tail(proper(X))
    11: cons(ok(X1),ok(X2)) -> ok(cons(X1,X2))
    12: tail(ok(X)) -> ok(tail(X))
    13: top(mark(X)) -> top(proper(X))
    14: top(ok(X)) -> top(active(X))
Number of Rules: 14
Direct POLO(Sum) ... failed.