(VAR X X1 X2 Y )
(RULES 
        active(f(g(X), Y)) -> mark(f(X, f(g(X), Y)))
        mark(f(X1, X2)) -> active(f(mark(X1), X2))
        mark(g(X)) -> active(g(mark(X)))
        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)
        g(mark(X)) -> g(X)
        g(active(X)) -> g(X)
        
)