anchor.diff:      If there is no node `u` in the graph this function will throw an `Error`.
   
       ```js
  -    var graph = new Digraph();
       digraph.addNode(1);
       digraph.node(1);
       // => undefined
  
positive.code: h2#Digraph Digraph
  :markdown
      The `Digraph` class represents a directed multi-digraph. To use this class
      use `require('graphlib').Digraph`.
  
  h3#Digraph-constructor new Digraph()
  :markdown
  
negative.code:     `target` are included. If the node `source` 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.inEdges(2);
      // => [ "A", "D" ]
  
      digraph.inEdges(2, 3);
      // => [ "D" ]
      ```
  
      Note that `digraph.inEdges(target, source)` yields the same result as
      `digraph.outEdges(source, target)`.
  
  h3#Digraph-outEdges digraph.outEdges(source, [target])
  :markdown
      Returns an array of ids for all edges in the graph that have the node
  