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