Nowadays, the old glory of SGML is almost forgotten as it got replaced by XML in all but a handful locations, but the language itself still has some very impressive capabilities that, I dare say, might yet make it a viable option for some very specific cases in the future. One of those features is "CONCUR", a.k.a. the original namespaces. This comparison is inaccurate of course, so I will first take a few paragraphs to describe what this feature is, and how it relates to the whole SGML ecosystem.
