I want to create a "frame" (not DS's frame of a mask) around a pattern, masking the portion of the pattern covered by the "frame".  E.g., I want to be able to tile a potion of the drawing area with hexagons, and view that tiling only of a rectangle.

Said another way, I want to show everything that is hidden my the MASK command, while hiding everything that is visible after the MASK command. Or alternatively, could I use the region between two polylines as the mask? I have jury rigged that by creating the mask I want away from my target geometry, filling it, and making the whole thing a block that I place over my target geometry (result shown below), but there should be an easier way!