Snapping a four-session losing streak, the Sensex at the Bombay Stock Exchange gained a modest 63 points, or 0.4 percent, to 15,518, on the first trading session of 2012 on Monday, on value-buying by investors. The Nifty at the National Stock Exchange rose 12 points, or 0.3 percent, to 4,637.Key stock markets in Japan , China , Hong Kong , and Singapore were all closed for a holiday.