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We considered and continue to consider all such currencies. Simplicity is prefered if it can be achieved. In this case.

Perhaps, rather than one currency, a basket could be used. That way any strong moves could be absorbed, and often countered by the opposing movement of another currency. It wouldn't have to be complicated, as long as the basket was set and not adjusted further unless there was a major event. In that case, there could be a rule in place that drops a currency if it drops by more than XX% or rises more than XX%.
I don't know if that's a good solution or not... just tossing it on the table.

Dan is allowing that the steem dollar may not absolutely equal "1 US dollar worth of steem". There is the the canadian dollar, the australian dollar, and maybe we can allow the Steem Dollar, as a distinct unit of account. To address the second black swan event. We could have it redeemable for: 12 Japanese Yen, 2 rmb, 0.3 dollars, and 0.3 euros worth of steem. So we have mitigated the effect of US dollar inflation. We need only for the witnesses to modify their price feed of the Steem Dollar, if that is what the blockchain settlement logic uses.