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However, Arthur is not necessarily a BPP verifier as it is not known whether MA is contained in the class .
I had two reasonds: first, it is nowhere explained what a BPP verifier is, so this sentence was coming out of the blue a bit. Second, it seems that for Arthur to be a BPP verifier, it suffices that ExistsBPP is contained in MA, which is known to be true. Therefore, the sentence seems to false too. However, without a proper definition of what it means to be a BPP verifier, I cannot be sure (coming back to the first issue), so I've removed the sentence instead of correcting it. Note that ExistsBPP is still mentioned in the 'Properties' section, where the statement seems to be correct.