anchor.diff:          (* Currently, we don't actually modify the definer, but this may
              change in future. *)
   
  +        (* This is needed for base downclosure checking.
  +           If it doesn't exist, then we don't do those checks. *)
  +        let empDefn =
  +            definer
  +            |> ViewDefiner.toSeq
  +            |> Seq.tryFind (fst >> List.isEmpty)
  +            |> Option.bind snd
  +
           let checkEach def definer =
  -            ok def >>= checkDef vprotos >>= (fun _ -> ok definer)
  +            ok def >>= checkDef empDefn vprotos >>= (fun _ -> ok definer)
   
           definer |> ViewDefiner.toSeq |> seqBind checkEach definer
   
  
positive.code:         ///     </para>
          /// </summary>
          | LookupError of name : string * err : Core.Instantiate.Types.Error
  
  
  /// <summary>
  
negative.code:     open Starling.Core.Pretty
      open Starling.Core.Expr.Pretty
      open Starling.Core.Symbolic.Pretty
      open Starling.Core.TypeSystem.Pretty
      open Starling.Core.Var.Pretty
      open Starling.Core.View.Pretty
  
  