https://github.com/w3c/activitystreams/issues/542
as:closed is defined as Object | Link | xsd:boolean | xsd:datetime
the normative context explicitly types the value to a datetime
solution: define closedBy
as a term definition for as:closed but with @type: @id
1 "as": "https://www.w3.org/ns/activitystreams#",
2 // ...
3 "closedBy": {
4 "@id": "as:closed",
5 "@type": "@id"
6 },
7 // ...