Grammar convergence is a lightweight verification method for establishing and maintaining the correspondence between grammar knowledge ingrained in all kinds of software artifacts, e.g., object models, XML schemas, parser descriptions, or language documents. The central idea is to extract grammars from diverse software artifacts, and to transform the grammars until they become syntactically identical. The present paper introduces and illustrates the basics of grammar convergence.
R. Lämmel, V. Zaytsev. An Introduction to Grammar Convergence. In Proceedings of 7th International Conference on Integrated Formal Methods (iFM 2009), LNCS 5423, pages 246–260. Springer, February 2009.

We describe a completed effort to recover the relationships between all the grammars that occur in the different versions of the Java Language Specification (JLS). The relationships are represented as grammar transformations that capture all accidental or intended differences between the JLS grammars. This process is mechanized and it is driven by simple measures of nominal or structural differences between any pair of grammars involved. Our work suggests a form of consistency management for the JLS in particular, and language specifications in general.
R. Lämmel, V. Zaytsev. Recovering Grammar Relationships for the Java Language Specification. In Proceedings of the 9th IEEE International Working Conference on Source Code Analysis and Manipulation (SCAM 2009), pages 178–186, IEEE, September 2009.