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