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(Paolo) I apologize that should have been a co:Set to be clearer. For Bags you need to have an instance of Item to support for repeated elements. Once you flatten down to co:element the repeated items become one. The idea of the co:element (defined as chain) in general is to provide a shortcut to the elements. In that case we are interested in finding the existence (one of them), the repetitions are irrelevant at that level.
(Rebecca) I see on the description of the Collections Ontology that "the Collections Ontology (CO) defines unordered collections (Set and Bag) and ordered collections (or List)." It seems awkward to me to select an ontology in which the type of the collection has to change if ordering is added to an originally unordered collection, or vice versa.
Example 4-ore: A Virtual Collection as an unordered collection of items using ORE Ontology's Aggregation with multiple unordered items
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