(VAR X X1 X2 )
(RULES 
        active(f(X, X)) -> mark(f(a, b))
        active(b) -> mark(a)
        mark(f(X1, X2)) -> active(f(mark(X1), X2))
        mark(a) -> active(a)
        mark(b) -> active(b)
        f(mark(X1), X2) -> f(X1, X2)
        f(X1, mark(X2)) -> f(X1, X2)
        f(active(X1), X2) -> f(X1, X2)
        f(X1, active(X2)) -> f(X1, X2)
        
)