What is the deal with subtracting solid 3d objects? It is a great idea, but it seems that the application remebers what has been subtracted. Every now and then I get a "ghost" rectangle or polygon that shows up when changing views. I know that these are the objects I drew to subtract from an existing piece. Occasionally, the odd shapes will actually replace the finished object At other times, an unwanted shape appears and it cannot be selected or deleted. What gives?