bug#64185: proposal for new function: copy-line

[Juri Linkov]

>> I do think it's more useful to put point on the second line; I almost
>> always find myself wanting to edit the "new" line, not the old
>> one. Yes, there isn't really a new vs old line, but I write the first
>> thing I want to see first, then want to duplicate it to make
>> a changed second line.

I agree that it's more useful to put point on the second line.
This is the reason why I can't use the new command 'duplicate-line',
so forced to continue using my own implementation in ~/.emacs.

> We could have the prefix arg of - (i.e., "C-u - M-x ...") produce that
> behavior from duplicate-line.

The problem is how to define this behavior with the prefix arg C-u in case
when a prefix numeric argument is used.

> Or we could have a user option to do that regardless of the argument,
> but then we should decide on which of the N new lines to put point:
> the first one copied or the last one.

A user option looks preferable.  In case of a prefix numeric argument,
it also makes sense to have 3 choices:

1. leave point on the old line
2. move point to the first new line
3. move point to the last new line