Stocks remained narrowly mixed Monday afternoon, with the tech-heavy Nasdaq holding a small gain. The Nasdaq rose 0.2%, thanks in part to a rebound in Apple (AAPL). The S&P 500 and the Dow Jones industrial average slipped 0.2% and 0.1%, respectively. Volume was tracking lower on the Nasdaq and a touch higher on the NYSE. A number of leaders were under pressure. 3D Systems (DDD) and Stratasys (SSYS), both makers of 3D printers and prototype