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I think pjmlp knows about this. The important word was "require", which makes a lot of difference. If you don't guarantee TCO at the language level, you can afford to emit code that performs other useful things (like debugging) while remaining conform to the language specification.

Then your code will become implementation dependent and fail spectacularly when trying to port it.

SBCL does TCE on some circumstances/optimization levels, not on every instance. Nor is it portable nor idiomatic CL. (Although I certainly write code that depends on it sometimes)