[edit] Apparently, I'm wrong, "utility" is an explicit requirement. But let me tell you, nobody ever asked me to justify the utility, not even a little bit. I think it's just assumed that if you're willing to pay the patent fees, it's likely to have some utility.
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AFAIK there's isn't. You can patent a leash for rodents, and PTO is not going to say "it's not acceptable, nobody wants to put rodents on a leash". The only questions are whether your invention is novel and non-obvious, not whether it's useful.
As such, if you can patent stage A that produces <unuseful stuff B>, nobody will tell you that it's unacceptable because B isn't useful. The usefulness is a completely separate question, to be decided by the market.
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AFAIK there's isn't. You can patent a leash for rodents, and PTO is not going to say "it's not acceptable, nobody wants to put rodents on a leash". The only questions are whether your invention is novel and non-obvious, not whether it's useful.
As such, if you can patent stage A that produces <unuseful stuff B>, nobody will tell you that it's unacceptable because B isn't useful. The usefulness is a completely separate question, to be decided by the market.