This object lets us persuade jackson into serializing JSOG structures. It is the id generated,
and a special serializer knows how to convert it to an @id or @ref as appropriate.
The ref value generated by other serializers might not be numeric, so we use String
used
public transient boolean used
A flag we use to determine if this ref has already been serialized. Because jackson calls the same
code for serializing both ids and refs, we simply assume the first use is an id and all subsequent
uses are refs.