anchor.diff:              let wsDir = Env.GetFolder Env.Folder.Workspace
               let res = repos |> Set.map (fun x -> let tag = if isHead then Tools.Vcs.Head wsDir x
                                                               else tagInfo.Format()
  -                                                 let hash = Tools.Vcs.TagToHash wsDir x tag
  +                                                 let hash = try
  +                                                                Tools.Vcs.TagToHash wsDir x tag
  +                                                            with
  +                                                                _ -> Tools.Vcs.Head wsDir x
  +
                                                    Bookmark(graph, x, hash))
               bookmarks <- Some res
   
  
positive.code: 
  // =====================================================================================================
  
  [<Sealed>] 
  type Factory(graph : Graph) = class end
  with
      let wsDir = Env.GetFolder Env.Folder.Workspace
  
negative.code: 
      member this.IsHead = isHead
  
      member this.Bookmarks : Bookmark set =
          collectBookmarks()
  
      static member (-) (ref : Baseline, target : Baseline) : Bookmark set =
  