Extents answer the question 'how much?' or 'how many?'. Fungible extents are normally represented by natural numbers. For instance, if we refer to "3 bucks", the "3" is the extent. Other extents may be represented as strings naming a particular right (e.g. seat 'G19'), or an arbitrary object that sensibly represents the rights at issue.
Extents must be pass-by-copy and Comparable. Extents are unlabeled, meaning that they alone are not necessarily associated with a particular assay or mint.