[CentOS] Unexpected behavior of 'yum group list' and 'yum group install'

Hi,

I'm currently experimenting with Yum on a fresh CentOS 7 minimal
install, and I'm getting some puzzling results. Here's what I did.

1. Install CentOS 7 from the Minimal CD.

2. Install 'deltarpm' and update all packages.

3. Install the "Core" package group: 'yum group install "Core"'

4. Install the "Base" package group: 'yum group install "Base"'

5. As expected, 'yum group list hidden | less' shows this:

     ...
     Installed Groups:
        Base
        Core
     ...

6. Now I remove manually all packages that were not present in the
initial installation. I'm doing this using a script, which is supposed
to get the system back to its pristine state. Various tests with 'rpm
-qa --queryformat '%{NAME}\n' | sort > packagelist.txt' show me that
this worked.

7. For mysterious reasons, 'yum group list hidden | less' still shows:

     ...
     Installed Groups:
        Base
        Core
     ...

8. When I try to reinstall the "Core" and/or the "Base" groups using
'yum group install "<Group>", I get the following result:

# yum group install "Core"
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * base: ftp.pasteur.fr
 * extras: mir01.syntis.net
 * updates: ftp.pasteur.fr
Maybe run: yum groups mark install (see man yum)
No packages in any requested group available to install or update

9. As you can guess, the expected behavior was for Yum to *not* display
the "Core" and "Base" groups as "Installed Groups" as soon as there was
some stuff (if not downright all packages from the group) missing.

Any suggestions on this?

Cheers from the rainy South of France,

Niki

PS: you may wonder why I'm doing this. I may have to manage a situation
where I have to deal with CentOS 7 installations that would first have
to be pruned down without reinstalling everything.

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