anchor.diff:          else 
               FrameworkIdentifier.KnownSpecialTargets 
               |> List.fold (fun (model : InstallModel) framework -> 
  -                   let g = framework.Group
  -                   { model with Groups = 
  -                                    Map.add g (match Map.tryFind g model.Groups with
  -                                               | Some group -> 
  -                                                   match Map.tryFind framework group.Frameworks with
  -                                                   | Some files when Set.isEmpty files.References -> 
  -                                                       { group with Frameworks = 
  -                                                                        Map.add framework { References = newFiles
  -                                                                                            ContentFiles = Set.empty } 
  -                                                                            group.Frameworks }
  -                                                   | None -> 
  -                                                       { group with Frameworks = 
  -                                                                        Map.add framework { References = newFiles
  -                                                                                            ContentFiles = Set.empty } 
  -                                                                            group.Frameworks }
  -                                                   | _ -> group
  -                                               | None -> 
  -                                                   { Frameworks = 
  -                                                         Map.add framework { References = newFiles
  -                                                                             ContentFiles = Set.empty } Map.empty
  -                                                     Fallbacks = InstallFiles.empty }) model.Groups }) this
  +                    model.AddOrReplaceGroup(
  +                        framework.Group,
  +                        (fun group ->
  +                            match Map.tryFind framework group.Frameworks with
  +                            | Some files when Set.isEmpty files.References -> 
  +                                { group with Frameworks = 
  +                                                Map.add framework { References = newFiles
  +                                                                    ContentFiles = Set.empty } 
  +                                                    group.Frameworks }
  +                            | None -> 
  +                                { group with Frameworks = 
  +                                                Map.add framework { References = newFiles
  +                                                                    ContentFiles = Set.empty } 
  +                                                    group.Frameworks }
  +                            | _ -> group),
  +                        (fun _ -> Some ({ Frameworks = Map.add framework { References = newFiles; ContentFiles = Set.empty } Map.empty
  +                                          Fallbacks = InstallFiles.empty })))) this
       
       member this.UseLastInGroupAsFallback() = 
           let groups = 
  
positive.code:         | Some x -> x.References
          | None -> Set.empty
  
  type InstallModel = 
      { PackageName : string
        PackageVersion : SemVerInfo
  
negative.code:             | NuspecReferences.All -> true
              | NuspecReferences.Explicit list -> List.exists lib.EndsWith list
  
          if not install then this else
  
          this.AddOrReplaceGroup(
              framework.Group,
              (fun group -> 
                  group.ReplaceFramework(
                      framework,
                      (fun files -> files.AddReference lib),
                      (fun _ -> Some (InstallFiles.singleton lib)))),
              (fun _ -> Some(FrameworkGroup.singleton(framework,InstallFiles.singleton lib))))
  
      member this.AddReferences(libs, references) : InstallModel = 
          Seq.fold (fun model lib -> 
  