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Periodically B, C and D are presented with a challenge from A, if they provide the correct solution, they are honest. You can classify this as a POW, and it makes up the 1st part of resource expenditure.
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Maybe this falls into the category of information you don't want to be pulic yet but I'll ask anyway.
Could you go into more detail on what such challanges might look like ? To me - obviously no expert in any sense of the word - it would seem difficult to make sure nodes aren't acting good-natured when answering challenges while acting evil when it comes to forming consensus.