partOf MUST contain list in the definition, but AnnotationCollection example doesn't

[robcast]

partOf in Prezi3 says it MUST contain a list https://iiif.io/api/presentation/3.0/#partof

In the example https://iiif.io/api/presentation/3.0/#58-annotation-collection the AnnotationPage's partOf does not contain a list.

Is that list really required? Can a structure e.g. an AnnotationPage be part of more than one other structure? In that case the example should be fixed.