Subsets and Acronyms
I just noticed: XHTML is a subset of both HTML and XML, but {H, T, M, L} and {X, M, L} are subsets of {X, H, T, M, L}. In fact, because of the way adjectives work, that’s always true. Add an adjective to something, and you get a subset of it (pizza, cheese pizza), but if you take their acronyms, it works the other way around. Neat.