Why post_number?

This allows Why post_number? - #3 by Kevin_Wildenradt to continue to work efficiently even if Why post_number? - #2 by sam is deleted.

And Why post_number? - #3 by Kevin_Wildenradt is arguably much prettier than Why post_number? - #9 by sam or Bootstrap failed with exit code 128 which does not hint properly on the topic.