'Is there a way to represent a conditional loop in an entity relations diagram?

So, I am designing a ERD for a renting outlet where the user can either borrow an item or make reservations, if the item is unavailable. I created two relationships namely; Borrow and Reserve between the USER entity and the ITEM entity. But, i figured there should be an additional loop indicating that, reservation should only be made if the item to be borrowed is currently UNAVAILABLE. Is there a way to represent this in a ERD? I am using the Chen notation.

sqldatabaseentity-relationship


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