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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.
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