Re: [PATCH v3] Allow applying filters to summary consecutively

[Gregory Heytings]

> > If I may, I would suggest something different: an option to query the 
> > user how they want the rmail-summary-by-* command to be applied: on the 
> > entire folder, to limit the existing summary, to extend the existing 
> > summary.
>
> The first of these three options is not useful -- to get that result, 
> just type q before the command.  So there would be only two that are 
> needed: union and intersection.

A third option would be more natural, I think.  After all, the first 
option is AFAIU the current effect of the rmail-summary-by-* command, they 
are applied on the entire folder and create a new summary.

> C-u would be a natural way to specify this.

C-u is already used by the rmail-summary-by-topic and 
rmail-summary-by-recipients commands for another purpose.