  Rule formats can quickly establish meta-theoretic properties of process
algebras. It is then desirable to identify domain-specific languages (DSLs)
that can easily express rule formats. In prior work, we have developed
Lang-n-Change, a DSL that includes convenient features for browsing language
definitions and retrieving information from them. In this paper, we use
Lang-n-Change to write a validator for the GSOS rule format, and we augment
Lang-n-Change with suitable macros on our way to do so. Our GSOS validator is
concise, and amounts to a few lines of code. We have used it to validate
several concurrency operators as adhering to the GSOS format. Moreover, our
code expresses the restrictions of the format declaratively.
