@quid I have added the tags you recommended. After all, there are many experienced users around who can correct the tags if they do not fit or can be improved in some way.

I see that I've mentioned above that matroids are mentioned in the tag-info for co.combinatorics but I forgot to add the link: mathoverflow.net/tags/co.combinatorics/info (Here is also link to the revision history.)

> Enumerative combinatorics, graph theory, order theory, posets, matroids, designs and other discrete structures. It also includes algebraic, analytic and probabilistic combinatorics.