Well... there's another thing... rather more complicated... but still works, plus gives you a lot of freedom to customize your selections
1. Normal selection: same as MS paint, once you select som other place, the current selection will dissapear
2. add to selection: when you select some area, it will become part of the selection
3. substract from selection: when you do some selection inside of the current selection, it will be unselected
4. intersect with current selection if you do a selection, and it touches or crosses the current selection, the resultant will be the common reas between selections
substract from selection may be of help in your case
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