Using an implementation as a definition has some advantages. All questions can be settled unambiguously by experiment. Debate is never needed, so answers are quick. Answers are always as precise as one wants, and they are always correct, by definition. Opposed to these one has a formidable set of disadvantages. The implementation may over-prescribe even the externals. Invalid syntax always produces some result; in a policed system that result is an invalidity indication and nothing more. In an unpoliced system all kinds of side effects may appear, and these may have been used by programmers.
