anchor.diff:  [<Test>]
   let ``should skip buckets which contain placeholder while adjusting upper versions``() = 
       let model = 
  -        InstallModell.FromLibs([ @"..\Rx-Main\lib\net20\Rx.dll"; @"..\Rx-Main\lib\net40\_._"; ])
  +        InstallModell.EmptyModel.Add([ @"..\Rx-Main\lib\net20\Rx.dll"; @"..\Rx-Main\lib\net40\_._"; ])
               .UseLowerVersionLibIfEmpty()
   
       model.GetFiles(DotNetFramework(Framework "v2.0", Full)) |> shouldContain @"..\Rx-Main\lib\net20\Rx.dll"
  
positive.code: [<Test>]
  let ``should inherit _._ files to higher frameworks``() = 
      let model = 
          InstallModell.FromLibs([ @"..\Rx-Main\lib\net40\_._"; @"..\Rx-Main\lib\net20\_._" ])
              .UseLowerVersionLibIfEmpty()
  
      model.GetFiles(DotNetFramework(Framework "v2.0", Full)) |> shouldContain @"..\Rx-Main\lib\net20\_._"
  
negative.code: 
  [<Test>]
  let ``should put _._ files into right buckets``() = 
      let model = InstallModell.EmptyModel.Add [ @"..\Rx-Main\lib\net40\_._"; @"..\Rx-Main\lib\net20\_._" ] 
  
      model.GetFiles(DotNetFramework(Framework "v2.0", Full)) |> shouldContain @"..\Rx-Main\lib\net20\_._"
      model.GetFiles(DotNetFramework(Framework "v4.0", Full)) |> shouldContain @"..\Rx-Main\lib\net40\_._"
  