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It's supposed to be impossible to prove a negative. But it might still happen some day. We just don't know.
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"it's impossible to prove a negative" is a simplification. A negation is just the oppositive of an affirmation. If the affirmation is "there is an element E of an infinite set S that satisfies property P", the negation would be "there is no E in S that satisfy P", which would make proving by enumeration require checking every element of an infinite set, which is impossible. But other forms of proof might be possible.

The set of US patents, however, are not infinite and, IIRC, is also public. That said, IP laws are a mess.

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Someone might also file a new parent, then apply it against RISC-V. You'd think that wouldn't be allowed to happen, and maybe it isn't, but only an expensive lawsuit will prove it
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Given the nature of the US legal system as based on common law, that applies beyond patents, and may affect ARM and x86 as well. In the end, the real, effective law is the one understood by judges, adjucated in court cases, built on precedents.

That being said, I don't expect someone filing a new patent after a RISC-V extension being published to last much longer beyond discovery in most cases, which should keep costs in lower end. Specially so in cases of bad faith.

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It was a joke I think.
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Well played
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