SEOUL, Dec 21 Daily net trading in shares on South Korea’s main stock exchange by three major categories of investors as of 0720 GMT, in billions of South Korean won : FOREIGNERS INSTITUTIONS RETAIL December 21 *78.1 -69.4 -7.7 ^December 20 60.6 -29.1 -33.5 December 19 5.0 -79.5 78.8 December 16