sig   type t =       [ `Acl       | `AltEditMedia       | `AltPost       | `Alternate       | `Batch       | `Edit       | `EditMedia       | `Feed       | `Icon       | `Parent       | `Post       | `ResumableCreateMedia       | `ResumableEditMedia       | `Revisions       | `Self       | `Thumbnail ]   val to_string : [> GdataDocumentsV3Model.Rel.t ] -> string end