"Anyway, and going back to the OP question: if , then and thus there's one unique function from or to the empty set."
@Tonio - Sorry but this is not making sense to me. As a funtion F, has to be a subset of AXB (which itself is empty) - so doesn't it mean it can't have a subset? Then in other words no such function exists. Maybe I'm not understanding it - is there a example you can give?