anchor.diff:              Set.add (source, target) edges)
   
       //
  +    member __.AddEdgeAndVertices (source : 'Vertex, target : 'Vertex) =
  +        // Preconditions
  +        // (None)
  +
  +        let vertices =
  +            vertices
  +            |> Set.add source
  +            |> Set.add target
  +
  +        VertexLabeledSparseDigraph (
  +            vertices,
  +            Set.add (source, target) edges)
  +
  +    //
       member __.RemoveVertex (vertex : 'Vertex) =
           // Preconditions
           if not <| Set.contains vertex vertices then
  
positive.code:         graph.AddEdge (source, target)
  
      //
      let inline removeVertex (vertex : 'Vertex) (graph : VertexLabeledSparseDigraph<'Vertex>) =
          graph.RemoveVertex vertex
  
  
negative.code:         graph.ContainsEdge (source, target)
  
      /// Creates a VertexLabeledSparseDigraph from a list of edges.
      let inline ofEdgeList (edgeList : ('Vertex * 'Vertex) list) =
          VertexLabeledSparseDigraph.OfEdgeList edgeList
  
      //
      let predecessors (graph : VertexLabeledSparseDigraph<'Vertex>) =
  