If the caller knows the difficulty of a PoW hash a given nonce
yields, it can tell the callee via the X-Hash-Difficulty, which
may allow the callee to skip some processing if the difficulty
does not meet some criterion.
In my case, a merge mining proxy can know it's pointless trying
to submit the nonce to a chain with higher difficulty when the
nonce only meets the difficulty for a lower difficulty chain.
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