anchor.diff:    // Implementation Methods // 
     //------------------------//
   
  +  /// Add an edge to this graph connecting [node1] and [node2].
     void addAdjacency(PointNode node1, PointNode node2) {
       if (this._adjacencyMap.containsKey(node1)) {
         this._adjacencyMap[node1].add(node2);
  
positive.code:     }
    }
  
    List<List<PointNode>> get adjacencyList {
      List<Set<PointNode>> adjacencySet = new List<Set<PointNode>>();
  
  
negative.code: part of path_finding;
  
  /// An undirected graph where each node is tied to a [Point].
  ///
  /// A UndirectedPointGraph can be used for most general
  /// purpose graph needs.
  class UndirectedPointGraph extends Graph {
    Map<PointNode, Set<PointNode>> _adjacencyMap =
      new Map<PointNode, Set<PointNode>>();
  
    /// Default constructor. Use .addAdjacency to populate this [Graph].
    UndirectedPointGraph() {
      
    }
  
    /// Conveinience constructor, that calls .addAdjacency for
    /// each element of [adjacencyList].
    UndirectedPointGraph.fromAdjacencyList(List<List<PointNode>> adjacencyList) {
      if (adjacencyList is! List) {
        throw new ArgumentError('Argument `adjacencyList` must be of type List!');
  