at property
At is the event's own instant, RFC 3339. It is part of the event's IDENTITY and not a filter: it is matched exactly, to the second, against the at the assertions were filed under, so an instant a second off names a different event and resolves to nothing. It is also what this event's as-of is measured from — At plus the horizon.
Please note: This property should have been non-nullable! Since the specification file does not include a default value (using the "default:" property), however, the generated source code must fall back to having a nullable type. Consider adding a "default:" property in the specification file to hide this note.
Implementation
String? at;