anchor.diff:                  // is the one we're trying to derive AND the parser position of this
                   // item is the initial position.
                   if item.Nonterminal = derivingNonterminal && item.Position = 0<_> then
  -                    Graph.addEdge (Item nonterminalDerivingItem) (Item item) () positionGraph
  +                    Graph.addEdge (Item nonterminalDerivingItem) (Item item) positionGraph
                   else
                       positionGraph))
   
  
positive.code:                     positionGraph
                      |> Graph.addVertex action
                      // Add an edge from this item to the action.
                      |> Graph.addEdge (Item item) action ()
                  else
                      match item.Production.[int item.Position] with
                      | Symbol.Nonterminal _ ->
  
negative.code:         elif not <| Set.contains target vertices then
              invalidArg "target" "The vertex is not in the graph's vertex-set."
  
          Set.contains (source, target) edges
  
      //
      member __.AddVertex (vertex : 'Vertex) =
          VertexLabeledSparseDigraph (
              Set.add vertex vertices,
              edges)
  
      //
      member __.AddEdge (source : 'Vertex, target : 'Vertex) =
          // Preconditions
          if not <| Set.contains source vertices then
              invalidArg "source" "The vertex is not in the graph's vertex-set."
          elif not <| Set.contains target vertices then
              invalidArg "target" "The vertex is not in the graph's vertex-set."
  
          VertexLabeledSparseDigraph (
              vertices,
              Set.add (source, target) edges)
  
      //
      member __.RemoveVertex (vertex : 'Vertex) =
  