Title: Numeric arguments in emacs

I'm starting an 'emacs trick of the day' sequence with:

- Function name: *universal-argument*
- Typical Key binding: `C-u`
- How to get help: `C-h f universal-argument`
- Usage: `C-u`

Receive a numeric argument that is given to the next called function,
when no numeric argument is typed, the value defaults to `4`. So today
you can try:

- `C-u 9 C-n`, that move cursor vertically down 9 lines
- `C-u C-k` which kills 4 lines
- `C-u C-u C-k` which kills 4 × 4, 16 lines
- `C-u 10 n` which enters `nnnnnnnnnn`

You may ask, what about if I want to input 25 '6' ? `C-u 256` can't work
...  so you just have to separate with another `C-u`: `C-u 25 C-u 1` gives
`6666666666666666666666666`.

Some functions does not have the simple 'repeating' effect of
receiving a numeric parameter, for example, running `C-u C-l` does not
recenter 4 times your screen! but the help page of
*recenter-top-bottom* states that:

> A prefix argument is handled like 'recenter':
> With numeric prefix ARG, move current line to window-line ARG.
> With plain `C-u`, move current line to window center. A negative argument to C-l move the current line to the line ARG from the bottom of the screen.