Keeps track of references from "origins" (things that refer) to "referents" (things that are
referred to), using an "original" collection of cross-references and a "new" collection. The utility
class can report whether a particular origin has been added or removed in going from the original
data to the new data, and whether a particular reference from an origin to a referent has been added
or removed in going from the original data to the new data.