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