Most Sector Leaders managed small gains Thursday, but a couple of stocks got hit hard. 3D Systems (DDD) extended its losing streak to five straight sessions. It slumped 3.78, or 9%, to 37.79 and sliced its 50-day line. There was no specific company news on the 3D printer maker, but Thursday's volume swelled 67% above average. The stock provides a good example of not buying stocks in a correction and following sell rules. 3D Systems has already had