The UML specs are a pain to work with. In theory you can start with emof (not that big) which is self-describing and self-hosting, then add a few features to get to cmof, then you can specify UML itself in cmof.

Trouble is that emof came after UML, and emof is defined on paper as "These features from UML 2 plus a few more"; whereas sane documentation would start out specifying emof, then layer the other specs on top of it.