anchor.diff:              // The set should have only one root vertex.
               match Set.count roots with
               | 0 ->
  -                Choice2Of2 "The graph is empty, or otherwise contains no root vertices."
  +                Choice2Of2 "The graph's vertex set is empty; or the graph is not a DAG."
               | 1 ->
                   Choice1Of2 <| Set.minElement roots
               | n ->
  
positive.code:             Choice2Of2 errorMsg
          | Choice1Of2 root ->
              /// The predecessors of each vertex.
              let predecessorsOf =
                  let initialPreds =
                      (Map.empty, graph.Vertices)
                      ||> Set.fold (fun initialMap vertex ->
                          Map.add vertex Set.empty initialMap)
  
                  (initialPreds, graph.Edges)
                  ||> Set.fold (fun predecessorsOf (source, target) ->
                      let predsOfTarget =
                          Map.find target predecessorsOf
                          // Add the source vertex to the predecessor set of target.
                          |> Set.add source
  
                      // Add the updated set to the map.
                      Map.add target predsOfTarget predecessorsOf)
  
              /// The graph vertices, excluding the root vertex.
              let vertices = Set.remove root graph.Vertices
  
negative.code:     let inline isEmpty (graph : VertexLabeledSparseDigraph<'Vertex>) =
          graph.IsEmpty
  
      (* TODO :   Implement a function to determine if the graph's edge-set is empty;
                  i.e., it may (or may not) have some vertices, but it does not have any edges. *)
  
      //
      let inline addVertex (vertex : 'Vertex) (graph : VertexLabeledSparseDigraph<'Vertex>) =
          graph.AddVertex vertex
  