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