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

[Gregory Heytings]

> > 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.

It seems more natural, at least to me, to ask the user how they want the 
command to be applied.  For example, currently C-M-r asks a single 
question:

Recipients to summarize by:

and each C-M-r invocation starts again from all messages in the file. 
Instead C-M-r could ask two questions, the first one being a multiple 
choice similar to y-or-n-p:

New summary/Limit current summary/Extend current summary (n or l or e):

> 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."

Did you bind 'q' to some other command in your local customizations, 
perhaps?  'q' is by default bound to rmail-quit/rmail-summary-quit, which 
quits Rmail.