anchor.diff:                      match fxWhen with
                       | Some x -> yield x
                       | None -> ()
  +
  +                yield XElement(NsMsBuild + "Otherwise",
  +                        new XElement(NsMsBuild + "PropertyGroup",
  +                            new XElement(NsMsBuild + packageWarnProp, "Y")))
           }
  -    if whens.Any() then XElement (NsMsBuild + "Choose", whens)
  -    else null
  +    
  +    seq {
  +        if whens.Any() then
  +            let targetName = sprintf "%s_Check" pkgProp
  +            yield XElement (NsMsBuild + "PropertyGroup",
  +                        XElement(NsMsBuild + "CompileDependsOn", sprintf "%s; $(CompileDependsOn)" targetName))
  +            yield XElement (NsMsBuild + "Choose", whens)
  +            yield XElement (NsMsBuild + "Target", 
  +                        XAttribute (NsNone + "Name", targetName),
  +                        XElement (NsMsBuild + "Error",
  +                            XAttribute(NsNone + "Text", sprintf "Warning! Can't find compatible package %A for requested FrameworkVersion $(FrameworkVersion)" package.toString),
  +                            XAttribute(NsNone + "Condition", sprintf "'$(%s)' != ''" packageWarnProp)))
  +    }
       
   let GenerateDependenciesContent (dependencies : PackageId seq) =
       seq {
  
positive.code:                       XAttribute(NsNone + "Condition", condition))
      }
  
  let GenerateProjectContent (package : PackageId) (imports : XElement seq) (choose : XElement) =
      let defineName = PackagePropertyName (package.toString)
      let propCondition = sprintf "'$(%s)' == ''" defineName
      let project = XElement (NsMsBuild + "Project",
  
negative.code:     let dependencies = NuGets.GetPackageDependencies xnuspec
  
      let imports = GenerateDependenciesContent dependencies
      let choose = GenerateChooseContent libDir package
      let project = GenerateProjectContent package imports choose
  
      let targetFile = pkgDir |> GetFile "package.targets" 
  