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