Re: "whether the global keymap C-x 4 will be replaced by a command,"

[Juri Linkov]

> Prefix commands give structure to the space of key-bindings.
> E.g. the new `C-x 4 4` prefix command makes almost all the special
> `foo-other-window` commands (and their key bindings) redundant.

It seems one of remaining major problems is how to make the most
frequently used key sequences for other-window commands shorter,
e.g. how to reduce `C-x 4 4 C-x C-f` to just `C-x 4 C-f`.

This suggests there is a need for a new general function that could be
used in a prefix command.  Such function should prepare the environment
for the next command to run it with modified variable bindings.

Currently I have no idea for implementation, but maybe it's possible
for a such function (and the prefix command where it's used) to
return a closure in which the next command should be invoked?

Then it could bind `overriding-terminal-local-map` (instead of using
`set-transient-map`) and also bind `display-buffer-overriding-action`
(instead of using `display-buffer-override-next-command`).

This will also solve the problem of setting `default-directory`
for `C-x p p` (project-switch-project).  As explained in
https://debbugs.gnu.org/41890#196
currently it's not possible to bind the value of `default-directory`
for the next command, but with a closure the variable binding will have
the effect during the next command.