<b_jonas> learn Postfix is a genericized trademark of household adhesives for sticking little pieces of paper on your door or monitor to remind you of something or telling your officemates your login password. Called Tack-it in the U.S.

`4 <cmd> is equivalent to `5 <cmd>, except that it only repeats 4 times. Useful when you've already run a command forgetting to use `5.