The SDR is a long way from the digital disruption that cypherpunks hope cryptocurrency can deliver. Dreamed up in the 1960s, SDRs are a kind of artificial currency whose value depends on other currencies. Dollars are a part of it, but so are euros, sterling, renminbi and yen. An SDR is a bit like a currency mutual fund for central banks. For decades it has been an afterthought in the global financial system, but the IMF has spent the past year thinking about how to give SDRs a broader...