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IIRC cleanroom engineering means you have to have a 'dirty' engineer thoroughly document the existing product, and then a 'clean' engineer has to look at the documentation and reproduce the behavior of the original product, without getting dirty by looking at it. True, the clean engineer could start at Bitshares with its free license which would be a good headstart. But it's not trivial, and the documentation might be a bit harder than it sounds.

Right I see, yes, as far as I can tell the graphene blockchain is open source, so I guess it would be building it up from the ground.

Difficult, but not impossible; in fact, I wouldn't be surprised if someone was doing it right now!

Cg