> If account a double spends x, they track where the coin goes, let’s say b accepts y:=x-? coin then a still has x-y spoiled coins and b now has y spoiled coins

> If account a double spends x, they track where the coin goes, let’s say b accepts y:=x-? coin then a still has x-y spoiled coins and b now has y spoiled coins. Now you just don’t accept transactions from accounts that have spoiled coins. That would be a perfect way to lock another user's account out of exchanges. Care to give me your address? I'd like to send you some spoiled bitcoins I have lying around... ;) As for the government, you can be sure they are already tracking all of the transactions. Bitcoin is far from anonymous - cash is a better alternative if you need anonimity, and even that can be traced easily nowadays.