I have had a nice and pedagogical idea to add to this answer of mine but I am afraid that, by doing an additional edit now, it may turn into a wiki-thing, as once happened.
I just want to do one additional edit. How can I know that? Yes, I know it is the policy of this site, but I prefer (as a personal taste I am not interested in discussing) not to edit it any more if it is going to be converted into an open answer.