I DO NOT KNOW what does being stable for addition mean: I said that if it is what ALLLL the rest of the world calls "closed under addition" then...etc.
Please read carefully my answer.
Raoh, this is YOUR problem. You used the term "stable under addition" which is not standard (English) so YOU are going to look in your text book or ask your teacher. Tonio asked what "stable under addition" meant and you responded by asking HIM, "Does it mean" and then gave, effectively, what in English would be called "closed under addition". I suspect that is correct, but I am afraid no one here can help you until YOU have checked the definition of "stable under addition".