(Bloomberg) -- The US Securities and Exchange Commission has issued a flurry of warning letters to some of the country’s most prolific providers of high-octane exchange-traded funds, effectively blocking the introduction of products designed to deliver three and even five times the daily returns of stocks, commodities and cryptocurrencies.In a set of nine almost identical letters posted Tuesday, the SEC told firms including Direxion, ProShares and Tidal that it would not move forward with reviewing proposed ...