code: public static <Vertex> DiGraph<Vertex> diGraph
  	(final Collection<Vertex> roots, final BiDiNavigator<Vertex> navigator) {
  	return new DiGraph<Vertex>() {
  	    public Collection<Vertex> getRoots() { return roots; }
  	    public BiDiNavigator<Vertex> getBiDiNavigator() { return navigator; }
  	};
      }
docs: Constructs a <code>DiGraph</code> object.
  
  @param roots Collection of root vertices.  This collection
  will be used directly, without being cloned or copied into
  another collection; none of the graph algoritrhms will mutate
  it.
  
  @param navigator Bi-directional digraph navigator