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.

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