anchor.diff:          graph.AddEdge (source, target)
   
       //
  +    let inline addEdgeAndVertices source target (graph : VertexLabeledSparseDigraph<'Vertex>) =
  +        graph.AddEdgeAndVertices (source, target)
  +
  +    //
       let inline removeVertex (vertex : 'Vertex) (graph : VertexLabeledSparseDigraph<'Vertex>) =
           graph.RemoveVertex vertex
   
  
positive.code:             Set.add (source, target) edges)
  
      //
      member __.RemoveVertex (vertex : 'Vertex) =
          // Preconditions
          if not <| Set.contains vertex vertices then
  
negative.code:         VertexLabeledSparseDigraph (
              vertices,
              reversedEdges)
  
      /// Creates a VertexLabeledSparseDigraph from a list of edges.
      static member OfEdgeList (edgeList : ('Vertex * 'Vertex) list) =
          (VertexLabeledSparseDigraph.Empty, edgeList)
          ||> List.fold (fun graph (source, target) ->
              graph.AddEdgeAndVertices (source, target))
  
      /// Creates a VertexLabeledSparseDigraph from a set of vertices.
      static member OfVertexSet (vertexSet : Set<'Vertex>) =
          VertexLabeledSparseDigraph (vertexSet, Set.empty)
  
  
  /// Functions on VertexLabeledSparseDigraphs.
  