anchor.diff:  :markdown
       Returns the ids of all edges in this digraph. Use
       [`digraph.edge(e)`](#Digraph-edge) to get the value for a specific edge.
  +
  +    ```js
  +    digraph.addNode(1);
  +    digraph.addNode(2);
  +    digraph.addEdge("A", 1, 2);
  +    digraph.addEdge("B", 1, 2);
  +    digraph.edges();
  +    // => ["A", "B"]
  +    ```
   
   h3#Digraph-eachEdge digraph.eachEdge(f)
   :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 `true` if the node with the id `u` is a member of the graph and
      `false` if not.
  
      ```js
      digraph.hasNode(1);
      // => false
  
      digraph.addNode(1);
      digraph.hasNode(1);
      // => true
      ```
  
  h3#Digraph-node digraph.node(u, [value])
  :markdown
      The `node` function provides a mechanism to get and set some user-defined
  