anchor.diff:    // Graph Interface Methods //
     //-------------------------//
   
  +  /// Returns every [PointNode] that is adjacent to [node].
     List<PointNode> getNeighbors(PointNode node, {bool onlyWalkable: true}) {
       Set<PointNode> neighbors = this._adjacencyMap[node];
       return neighbors.toList();
  
positive.code: part of path_finding;
  
  class UndirectedPointGraph extends Graph {
    Map<PointNode, Set<PointNode>> _adjacencyMap =
      new Map<PointNode, Set<PointNode>>();
  
    UndirectedPointGraph() {
      
    }
  
    UndirectedPointGraph.fromAdjacencyList(List<List<PointNode>> adjacencyList) {
      if (adjacencyList is! List) {
        throw new ArgumentError('Argument `adjacencyList` must be of type List!');
  
negative.code:     }
    }
  
    /// Return a ListList<PointNode>> where each element is an
    /// [PointNode] pair representing an edge of this graph.
    List<List<PointNode>> get adjacencyList {
      List<Set<PointNode>> adjacencySet = new List<Set<PointNode>>();
  
  