Add the chairman of the largest electronic brokerage firm in the U.S. to the list of Cassandras warning us of the perils posed by the trading of cryptocurrencies. In a remarkable open letter in The Wall Street Journal addressed to J. Christopher Giancarlo, the Chairman of the Commodity Futures Trading Commission, Thomas Peterffy, Chairman of Interactive Brokers, reacted adversely to a proposal by the Chicago Mercantile Exchange to allow Bitcoin and other cryptocurrency deriva