Snapping three days of gains, the Sensex at the Bombay Stock Exchange ended with a small loss of 13 points, or 0.1 percent, at 20,148, on profit-booking by investors ahead of the monthly settlement in the derivatives segment, and as European markets fell, today. The Sensex had gained 487 points in the previous three trading sessions. The Nifty at the National Stock Exchange dropped 7 points or 0.1 percent, to 6,104.Stock markets in Japan, China, Hong Kong, South Korea and Singapore ended mixed.