$ apt modernize-sources
...
For a simulation, respond N in the following prompt.
Rewrite 3 sources? [Y/n] N

First, the N should also be inside the box, as it is a valid choice. But
then it competes with the Y for looking like the default. So use "s" to
simulate.

Then, what if the user only wants to mess with their second out of three
sources. So please say there how to pick other than just "all or
nothing".

Maybe [123][Yns]?

What about if they have more than nine? Well they must be experts
already.

Control: forcemerge 1094594 -1

The prompt is what it is, it's a simply Y/n prompt used everywhere, we
don't have much choices here.

Not going to happen. I'm going to merge it with the other one that
requests essentially a stdin | stdout piping ability, that will give
you selective output.