anchor.diff:              (fun group -> 
                   group.ReplaceFramework(
                       framework,
  +                    (fun _ -> InstallFiles.singleton lib),
                       (fun files -> files.AddReference lib),
                       (fun _ -> Some (InstallFiles.singleton lib)))),
               (fun _ -> Some(FrameworkGroup.singleton(framework,InstallFiles.singleton lib))))
  
positive.code:             (fun group ->
                  group.ReplaceFramework(
                      framework,
                      (fun files -> files.AddFrameworkAssemblyReference assemblyName),
                      (fun _ -> Some (InstallFiles.empty.AddFrameworkAssemblyReference assemblyName)))),
              (fun _ -> None))
  
negative.code:                     model.AddOrReplaceGroup(
                          framework.Group,
                          (fun group ->
                              group.ReplaceFramework(
                                  framework,
                                  (fun _ -> { References = newFiles; ContentFiles = Set.empty }),
                                  (fun files -> files),
                                  (fun _ -> Some({ References = newFiles; ContentFiles = Set.empty })))),
                          (fun _ -> Some(FrameworkGroup.singleton(framework,{ References = newFiles; ContentFiles = Set.empty }))))) this
      
      member this.UseLastInGroupAsFallback() = 
  