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It seems to me that an "open platform" should allow a clean-room re-implementation, *and* for you to implement as little or as much as you like, or indeed to modify it in any way that you like

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What does "clean room re-implementation" re-implementation actually mean?.

(a) Thinking about an abstract problem, then designing and implementing a new solution from first principles without reference to any existing solution?

(b) Copying some lines of code, but not all of it.

I can see that (b) could be covered under a "fair use" clause, but it cannot possibly be "clean room"