bug#54977: 29.0.50; Customising eshell-modules-list means you won't get new default entries

[Jim Porter <jporterbugs@gmail.com>]

On 4/16/2022 1:55 PM, Sean Whitton wrote:
> Suppose that you use the customisation interface to enable the
> eshell-rebind module, and save the result in your init.  Now upstream
> Emacs gains a new on-by-default module, e.g. the recently added
> em-extpipe.  While everyone who hasn't customised eshell-modules-list
> gets the new module automatically, you'll have to customise the variable
> again, and you can't even just tick a box to add it, as with the other
> truly optional modules, but must manually insert it at the end of the
> list.  This is no good.

Generally I resolve this in my config by using a combination of 
`add-to-list' and `delete' to manually customize lists like this. Maybe 
it would be good to provide something like that for users of the 
Customize interface as well? Then any defcustom that takes a list could 
either be customized to have an exact set of elements (as it is today), 
or to have "the default, but with FOO added and BAR removed".