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

[Richard Stallman]

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  > >> 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.

I don't follow the reasoning.

                                                    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.

That is true -- but what does it have to do with what user interface
to use for choosing between union and intersection here?
I don't see how it relates.

My point is that we already have q as a way to specify to disregard
the previous filtering and start from "all the messages in the file."

  > > 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.

Indeed it is.  Hmm.

Nonetheless, prompting for the choice of set operation
is not very convenient.  It would be nicer to have something
the user can type before the filtering command to specify how
to combine it.

Here's an idea.  Type & to do intersection with the following filter,
and type | to do union with the following filter.  Both & and | are
undefined in Rmail mode, so they are available for this.  They
will be easy to remember, too.

-- 
Dr Richard Stallman (https://stallman.org)
Chief GNUisance of the GNU Project (https://gnu.org)
Founder, Free Software Foundation (https://fsf.org)
Internet Hall-of-Famer (https://internethalloffame.org)