anchor.diff:  let allPrereleases versions = versions |> List.filter (fun v -> v.PreRelease <> None) = versions
   
   let cleanupNames (model : PackageResolution) : PackageResolution = 
  -    model |> Seq.fold (fun map x -> 
  -                 let package = x.Value
  -                 let cleanup = 
  -                     { package with Dependencies = 
  -                                        package.Dependencies 
  -                                        |> Set.map (fun (name, v, d) -> model.[name].Name, v, d) }
  -                 Map.add package.Name cleanup map) Map.empty
  +    model
  +    |> Map.map (fun _ package ->
  +        { package with 
  +            Dependencies = 
  +                package.Dependencies 
  +                |> Set.map (fun (name, v, d) -> model.[name].Name, v, d) })
   
   [<RequireQualifiedAccess>]
   type Resolution =
  
positive.code:     ResolvedSourceFiles : ModuleResolver.ResolvedSourceFile list }
  
  type UpdateMode =
      | UpdatePackage of  GroupName * PackageName
      | UpdateGroup of GroupName
      | Install
      | UpdateAll
  
negative.code:             | true,Resolution.Conflict(_) -> tryToImprove true |> snd
              | _,x-> x
  
      step (Map.empty, [], Set.empty, rootDependencies)