Virtual currencies : the oversight role of the U.S. Securities and Exchange Commission and the U.S. Commodity Futures Trading Commission : hearing before the Committee on Banking, Housing, and Urban Affairs, United States Senate, One Hundred Fifteenth Congress, second session, on examining the growing world of virtual currencies and the oversight conducted by the U.S. Securities and Exchange Commission and the U.S. Commodity Futures Trading Commission, February 6, 2018