TIMEOUT
MAYBE
Trs:
 {            mark(a()) -> active(a()),
              mark(b()) -> active(b()),
    mark(f(X1, X2, X3)) -> active(f(X1, mark(X2), X3)),
            active(b()) -> mark(a()),
   active(f(a(), X, X)) -> mark(f(X, b(), b())),
    f(mark(X1), X2, X3) -> f(X1, X2, X3),
  f(active(X1), X2, X3) -> f(X1, X2, X3),
    f(X1, mark(X2), X3) -> f(X1, X2, X3),
  f(X1, active(X2), X3) -> f(X1, X2, X3),
    f(X1, X2, mark(X3)) -> f(X1, X2, X3),
  f(X1, X2, active(X3)) -> f(X1, X2, X3)}
 Open