More sophisticated rules seeking to prevent another “flash crash” like the one that rattled the markets on May 6, 2010 take effect today, but observers urge regulators not to declare mission accomplished yet. Specifically, new so-called “limit up/limit down” rules will initially require trades of a group of liquid listed stocks be executed within a range tied to recent prices for that security. Later, all exchange-traded stocks will be subject to the rules. They replace a more blunt circuit-breaker program the Securities and Exchange Commission adopted in May 2011, a year after the flash crash sent the Dow Jones ... (full story)