When validating geometries, what are "Nested shells"?

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I am validating shapefiles polygon geometry using shapely, and have also attempted it using FME. Some polygons return the error "Nested Shells".

I cannot find a definition of what the problem is, whether in FME documentation (here) or in google search results on shapely or OGC geometry validation.

Would anyone have a definition or (ideally!) a graphic example? Many thanks!!



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