to prove you're not having peers maliciously omit UTXOs. ...but iirc, I think Electrum suffers that flaw, right? It's only the old school SPV wallets that are immune.
proving absence has always been a problem, it's a solvable problem tho with adjusted tree structure (like merklix tree - kinda useless in its original intention for transaction, but may be useful elsewhere, especially on a global tree like utxo)
I'll have to refresh on that one too. I remember I've looked it up before but have definitely forgotten.
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