This is a far away feature request I would like to discuss here. If we could plan more detailes together I would summarize them later in an Issue on GitHub if you want. Currently I am using JabRef 3.8. Of course I would wait until 4.x is stable.
I would like to connect specific groups with boolean operators (AND, OR, NOT). When "connecting" two groups this way JabRef should create a new result-group with the resulting entries. For the reason that a scientific research process need to be documented/transparent the entries should be duplicated for real. So don't add just a new group name to the groups-field. Copy it for real. Of course this behaviour should be customizable.
Background of my needs:
Usual you would do this in a database search front-end (e.g. PubMed) of course.But sometimes they don't offer features like that. Having a literature software offering this features would not only be a great backup for situations like that. It would be a real value for the scientific community I think.