Depends on D1846.
Very basic implementation of support for a "currency"-system. (Petra-AI complies.) All barterable resources can only be sold into the "currency"-resource(s).
This should be extended to something like:
Resource A can be bartered into [Resource B, Resource C].
Resource B can be bartered into [Resource A, Resource D].
Resource C can be bartered into [Resource D].
Resource D can be bartered into [Resource A, Resource B, Resource C].
for maximum moddability.
For now it is mainly a reference for @wowgetoffyourcellphone and @Nescio.