> But there’s nothing stopping you from re-coding existing implementations of GPL’ed code.

There is if you've read the code. You can still violate copyright if you're hand-copying an implementation of a feature.

Lawyers recommend "clean room" re-implementations when reproducing copyrighted functionality (when folks bother to ask) precisely to avoid the risk of code being copied verbatim.

Functionality can be copied. Code can not.

Are you sure you can do that? Because it is trivial to paraphrase code and text.

Sure. I’ve done it for Bel.

  (def no (x)
    (id x nil))

  (def atom (x)
    (no (id (type x) ‘pair)))

  (def some (x f)
    (if (no x) nil
        (f (car x)) x
        (some (cdr x))))

  (def all (x)
    (if (no x) t
        (f (car x)) (all (cdr x))
        nil)))

I don’t even have to pull up bel.bel to know that those are almost perfect replicas. I typed it on my iPad.

EDIT: as far as I can tell, the only diff is that all comes before some. https://sep.yimg.com/ty/cdn/paulgraham/bel.bel?t=1595850613&

I could have kept going through most of the implementation.

I'd wager that most folks would consider the code for some basic list operations to be a little different from outputting a complete specific implementation of a fast square root function with its exact constants, including the comments.

Why? They’re both basic functions. I’d argue that if pg had left comments and constants, I’d have replicated those too.

Perhaps my example isn’t as impressive as the model’s capability, but operationally it’s the same.