RE: [found the culprit]

[Drew Adams <drew.adams@oracle.com>]

> >>  Compress or uncompress marked (or next prefix argument) files.
> >>  A prefix argument ARG specifies files to use instead of marked.
> >>   An integer means use the next ARG files (previous -ARG, if < 0).
> >>   `C-u': Use the current file (whether or not any are marked).
> >>   `C-u C-u': Use all files in Dired, except directories.
> >>   `C-u C-u C-u': Use all files and directories, except `.' and `..'.
> >>   `C-u C-u C-u C-u': Use all files and all directories.
> >
> > Is "C-u 0' available?
> 
> I don't think we should base our decision on what some external package
> happened to choose.  Especially since the above C-u usage does not
> follow Emacs's use of C-u.

1. I didn't say you should base your decision on what Dired+
uses.  I just mentioned what it uses, and has used for a
long time.  And it's a general scheme, applied to `dired-do-*'
commands generally.  The conflict is not a minor one, e.g.,
affecting just `Z'.

A better idea is for vanilla Emacs to adopt that general
scheme.  It's a good scheme - I didn't just "happen to
choose" it.  And it fits with the existing vanilla behavior,
both wrt numeric arg and plain C-u.  The only divergence is
the addition of behaviors for multiple C-u, which most users
will use much less commonly.  (Well, actually, I find I use
multiple C-u more than I use numeric.)

2. How does the above C-u usage "not follow Emacs's use of C-u"?

3. `Z' is a `dired-do-*' command.  Its prefix arg behavior
should, other things being equal, follow the pattern of
the other `dired-do-*' commands.

However, the behavior of `Z' itself (`dired-do-compress')
can be whatever you want, including treating directories
specially.