||ITEMs are often grouped together so that they can be managed, or operated upon together. For example
* Housing Stock
* Vehicle Fleet
* a Contracts Register
COLLECTIONs are defined by AGENTs so that the actual contents are either explicitly listed, or defined by a scoping statement.
COLLECTIONs enable lists of ITEMs to be defined, created and shared across a city.
Although the contents of a COLLECTION can be physical ITEMs, a COLLECTION is an ABSTRACT ITEM.
As a COLLECTION is itself, a sub-concept of ITEM, it's STATE can be described and tracked over time.