“ How does one prove a negative?”
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That’s not a “negative.”
They said nobody is shedding graphene. All one would need do is cite the credible evidence they obviously believe that makes their claim valid. In other words, that evidence that the persons with the graphene in their blood cannot shed graphene nanoparticles. Or prove it themselves with scientific reason.
You have to prove this assertion that such a thing is scientifically possible.