anchor.diff:      the results to be filtered such that only edges between `u` and `v` are
       included in the returned array. If the node `v` is specified but not a
       member of the graph this function raises an `Error`.
  +
  +    ```js
  +    digraph.addNode(1);
  +    digraph.addNode(2);
  +    digraph.addNode(3);
  +    digraph.addEdge("A", 1, 2);
  +    digraph.addEdge("B", 2, 3);
  +    digraph.addEdge("C", 3, 1);
  +    digraph.addEdge("D", 3, 2);
  +
  +    digraph.incidentEdges(3);
  +    // => [ "B", "C", "D" ]
  +
  +    digraph.inEdges(3, 2);
  +    // => [ "B", "D" ]
  +    ```
   
   h3#Digraph-equals digraph.equals(other)
   :markdown
  
positive.code:     Returns the number of nodes in the digraph. For example:
  
      ```js
      var graph = new Digraph();
      digraph.addNode(1);
      digraph.addNode(2);
  
  
negative.code:     Returns the number of edges in the digraph. For example:
  
      ```js
      digraph.size();
      // => 0
  
      digraph.addNode(1);
      digraph.addNode(2);
      digraph.addEdge(null, 1, 2);
  