Optional activity
activity?: NamedNode<"https://www.w3.org/ns/activitystreams#Create"> | NamedNode<"https://www.w3.org/ns/activitystreams#object"> | NamedNode<"https://www.w3.org/ns/activitystreams#Add"> | NamedNode<"https://www.w3.org/ns/activitystreams#Delete"> | NamedNode<"https://www.w3.org/ns/activitystreams#Remove"> | NamedNode<"https://www.w3.org/ns/activitystreams#Update">