does a preview object have to have it's own unique ID: https://www.w3.org/TR/activitystreams-vocabulary/#dfn-preview ???
/cc @evan @trwnh @johnonolan
does a preview object have to have it's own unique ID: https://www.w3.org/TR/activitystreams-vocabulary/#dfn-preview ???
/cc @evan @trwnh @johnonolan