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this could be kinda nice. a group for apps that have Linux versions, dev tools, open source only, etc.
"The option should exist to be able to select only from applications that have Linux versions, as a means of facilitating an OS transition."
Yes, that is an interesting idea. I've proposed that as transition strategy a while ago.
The metalink spec has a "tags" tag that could be used to implement this. Alternatively separate "groups" might be defined in apps.xml. This gives more flexibility on the user end of making custom groups. This can then be implemented in Appupdater without too much trouble. This is similar to the comps.xml file that redhat/fedora uses during installs. Limiting to specific groups might be more difficult, but doable.
As far as open source goes, that should be pretty easy to do. I've been including license information in my metalink files for a long time, so just checking the license should be sufficient.
I just checked in some code that contains basic group support. It is currently available in the command line version only. It allows you to install/remove and do some other things by group. I haven't added anything yet to make this an "enforceable" policy where certain groups cannot be installed.