The current model for the STRUCTURE classes means that there is an unnecessary intervening CLUSTER in instance structures for the ITEM_LIST type, due to the representation attribute being only 1:1.
Change the model to have efficient representations for all
the data structure types (but still based on CLUSTER and ELEMENT),
including ITEM_LIST. Show in the text how these structures would
be coded in CEN ENV 13606 structures. Add a function
as_hierarchy:ITEM to DATA_STRUCTURE, and describe how this would
be implemented in all subtypes of DATA_STRUCTURE.