(VAR X X1 X2 X3 )
(RULES 
        active(f(a, X, X)) -> mark(f(X, b, b))
        active(b) -> mark(a)
        mark(f(X1, X2, X3)) -> active(f(X1, mark(X2), X3))
        mark(a) -> active(a)
        mark(b) -> active(b)
        f(mark(X1), X2, X3) -> f(X1, X2, X3)
        f(X1, mark(X2), X3) -> f(X1, X2, X3)
        f(X1, X2, mark(X3)) -> f(X1, X2, X3)
        f(active(X1), X2, X3) -> f(X1, X2, X3)
        f(X1, active(X2), X3) -> f(X1, X2, X3)
        f(X1, X2, active(X3)) -> f(X1, X2, X3)
        
)