I think you do need "bug for bug" compatibility between semantics—because the other choice is worse—but that drives home what it takes to genuinely have alterna

I think you do need "bug for bug" compatibility between semantics—because the other choice is worse—but that drives home what it takes to genuinely have alternative semantic choices: sufficient concision of definition to be able to maintain the choices! This is standard for programmers, though. It's architecture like any other.