I don't know if it's slower or not. It takes a certain amount of time to develop a functional spec; you have a head start if you have a working example in fron

I don't know if it's slower or not. It takes a certain amount of time to develop a functional spec; you have a head start if you have a working example in front of you. I think the main reason you don't see more clean-room reimplementations is simply, as you say, that level of compatibility is rarely necessary -- indeed, it's often considered undesirable, even by the users.