Re: master 49f3888c5dc: Add new options to 'query-replace-show-preview' (bug#81583)

>>     * lisp/replace.el (query-replace-show-preview): Accept 'both'
>>     and a function of two arguments as values.  Doc fix.
>
> With these user options with multiple possibilities, where there isn't a
> backwards compatibility issue to consider (as there isn't here), it's
> nice to make it so that `t' turns on all the features and the other
> values turn them on partially.  Then there is a natural ordering
> nil->something->other->t.
>
> So could we perhaps make `t' mean `both' and use another symbol for when
> only the result is displayed?

Another variant is to support either nil or a value with
a function also for the case when only the result is displayed
with e.g. 'replace-preview-only-replacement' instead of t.