anchor.diff:  open System.Collections.Generic
   open FSharp.FGL //For our Graph method
   
  -//  An immutable directed graph data structure efficient for large sparse
  -//  graph representation where out-edge need to be enumerated only.
  -//  Adaptation of https://github.com/YaccConstructor/QuickGraph.
  +
   
   module internal Dictionary = 
   
  
positive.code:            this.RemoveEdge edge |> ignore
         this
      
      //
      //Vertices
      ///Returns true, if the graph contains the vertex, else false.
      member this.ContainsVertex(vertex:'Vertex) :bool =
  
negative.code:          |Some x -> x
           |None   -> failwithf "The vertex %O does not exist in the graph." v
  
      //Label
      ///Returns Some label, if a label for the vertex v exists, else none.
      member this.TryGetLabel(v:'Vertex) :'Label option =        
  