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1. Simple contours: The contours are simple closed curves.
2. Unique contour labels: Each contour has a unique label.
3. No concurrency: Contours are not concurrent (that is they meet at a discrete set of points).
4. No tangential intersections: Contours do not touch, but can cross each other transversely wherever they meet.
5. No multiple points: At most two contours meet at a single point.
6. Unique zone labels: Each zone can be uniquely identified by the set of contours containing it and the set of contours excluding it.