> You still have to transfer the same amount of information to the computer. Different target languages to transfer the information to may result in somewhat u

> You still have to transfer the same amount of information to the computer. Different target languages to transfer the information to may result in somewhat uncorrelated defects, allowing finding defects through mutual comparison. You can test a generator by writing a recognizer and vice versa. Errors in formal proof of correctness are somewhat uncorrelated with errors made in tests and errors made in implementation. By doing all 3 you can diversify to reduce risk.