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