anchor.diff:  module Fake.MSBuild.ProjectSystem
   
   open Fake
  +open System.Collections.Generic
   open System.Xml
   open System.Xml.Linq
   open XMLHelper
  
positive.code:         let document = XMLDoc documentContent // we create a copy and work immutable
          let node = 
              getCompileNodes document 
              |> List.filter (fun node -> node.Attributes.["Include"].InnerText = fileName) 
              |> Seq.last  // we remove the last one to make easier to remove duplicates
  
          node.ParentNode.RemoveChild node |> ignore
  
negative.code:         new ProjectFile(projectFileName,document.OuterXml)
  
      /// All files which are in "Compile" sections
      member x.Files = getCompileNodes document |> List.map getFileAttribute
  
      /// Finds duplicate files which are in "Compile" sections
      member x.FindDuplicateFiles() = 
          [let d = Dictionary()
           for i in x.Files do
              match d.TryGetValue(i) with
              | false,_    -> d.[i] <- false         // first observance
  