anchor.diff:      `target` are included. If the node `target` is specified but is not in the
       graph then 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.outEdges(3);
  +    // => [ "C", "D" ]
  +
  +    digraph.inEdges(3, 2);
  +    // => [ "D" ]
  +    ```
  +
   h3#Digraph-incidentEdges digraph.incidentEdges(u, [v])
   :markdown
       Returns an arrray of ids for all edges in the graph that are incident on
  
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:     If there is no node `u` in the graph this function will throw an `Error`.
  
      ```js
      digraph.addNode(1);
      digraph.node(1);
      // => undefined
  