If we consider Panini's grammar with it's four thousand rules and lists as a theory of the Sanskrit language (just as Newton's mechanics is a throy of the dynamics of physical objects) then we can test it. There are multitudes of corroborations, and a few wrong predictions. The paribhasas are the extra rules to correct the predictions. Introduces linguists to some of the complex issues in getting one of the best theories of grammar ever to actually work.