There are a lot of areas where most Common Lisp implementations have converged on a common way to do something, even though the specification allows freedom to approach them in divergent ways.

For example, it’s not required that (code-char 97) evaluates to #\a , but in practice, that’s how all implementations currently work.

Similarly, although probe-file’s description repeatedly refers to a file, in practice, nearly every implementation allows a directory pathname. (CLISP signals an error, instead, and has a parallel set of system functions that can probe directories.)

And although it’s not required, reading and writing specialized octet vectors to streams does what you’d generally expect. The octets on the Lisp side line up one-to-one with the octets that are read from or written to the stream. There isn’t any padding or swapping or headers or other stuff.

I think it would be pretty cool to accumulate these de facto standard behaviors into a test suite so you could see exactly where you need to adjust your expectations, and perhaps provide persuasive evidence to change unnecessarily divergent behavior.

I don’t have time to make this, sorry! But if you make it, let me know, because I’d love to try it out.