Support require one or the other nature

Currently one can only require another nature by id, or one can only allow one or the other nature.
What I recently came along (and seem not supported), is that one wants to require any of a set of natures (e.g. nature `a` or nature `b`) because both are exclusive (you can only have `a` or `b` on the project) but my nature wants at least one of them.

So from an extension point of view I then need to mark one as the "preferred" that is added if none of the ones I want are available.