Quote Originally Posted by SergeMak View Post
I wouldn't call it "rule", rather "set expression". E.g. "А не выпить ли нам коньячку?"
There are of course other means to express this idea, e.g.: "может быть, пойдем по лестнице?", "может быть, по коньячку?", "почему бы нам не пойти по лестнице?", and so on.
How many years do you think I need to learn to speak like this?
I think the answer is simple: Never.