I must have hit the wrong button or something, but a small, black arrow has appeared on the left side of my source code window. When I click it, it becomes highlighted in red, and when I click it again, the highlight is removed. I can't seem to find any way to delete or undo it. Worst of all, my code won't run! The class compiles without reporting any errors, but the world screen remains opaque. Can someone please help me? Thank you in advance.