Z ep sugar comes in paper sacks. T he ZEP mark is prominently and unmistakably used as a trademark on the sacks. T he sacks also carry a beautiful geometric design, quite different from any other sugar bag on the market. T he design is quite subtle, however, and sugar buyers pay it little attention. T o the extent consumers notice the design at all, they regard it as ornamental and do not associate it with any particular brand of sugar. A rival of Zep begins to use the design on its own bags of sugar. Zep argues that the design is a trademark because it appears only on Zep sugar. I s the design a source-identifying symbol?