Again, thank you for your patience and your reviews.


PS: the prompt now looks like this ('?' characters are highlighted with
the warning face):

With markers:

> Confirm:
> sed 's/\?/!/'
>         ^
> Send 1 occurrence of ‘?’ as-is to shell?

Without markers:

> Confirm:
> sed 's/\?/!/'
> Send 1 occurrence of ‘?’ as-is to shell?

I added the "Confirm:" line because

- y-or-n-p adds "Please answer y or n.  " before the prompt when the
  user fails to answer correctly, so the markers would not line up if
  the command remained on the first line,

- y-or-n-p adds " (y or n)" after the prompt; I find it more legible to
  have the question next to this suffix, so I did not want to move the
  question to the first line.