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