A Proxy representing a single item associated with a numeric index.

The item of a Slot provides the thing being it proxies.

When an Ordered Aggregation has a slot, its index must be unique within that Ordered Aggregation. However, Slots are permitted to be proxies in other kinds of Aggregation, in which case index uniqueness is not required.

When a Slot has an ordered list, its index should be less than or equal to the length of that Ordered List.