public static void main(String[] args)
	{
		
		SymbolDigraph<String> sg = new SymbolDigraph<String>(4);
		try
		{
			sg.addEdge("Main", "Object"); 
			sg.addEdge("A", "B");
			sg.addEdge("B", "A");
		}
		catch (Exception e)
		{
			e.printStackTrace();
		}

		System.out.println(sg);
		System.out.println(sg.keys);
	}