anchor.diff:  ///     <paramref name="graph" /> and its view is structurally equal
   ///     to <paramref name="nodeView" />.
   /// </returns>
  -let nodeHasView nodeName nodeView graph =
  +let nodeHasView (nodeName : NodeID) (nodeView : SVGView) (graph : Graph)
  +  : bool =
       match (Map.tryFind nodeName graph.Contents) with
       | Some (InnerView v, _, _, _) -> v = nodeView
       | _ -> false
  
positive.code:         { /// <summary>
            ///     The name of the edge.
            /// </summary>
            Name : string
            /// <summary>
            ///     The name of the source node.
            /// </summary>
            SrcName : string
            /// <summary>
            ///     The view of the source node.
            /// </summary>
  
negative.code:     ///     A pretty-printer <c>Command</c> representing
      ///     <paramref name="_arg1" />.
      /// </returns>
      let printEdge (id : EdgeID) ({ Pre = s; Post = t; Cmd = cmd } : Edge)
        : Doc =
          hsep [ s |> String
                 String "->"
                 t |> String
  