The DigraphAsMatrix class can be implemented
	by extending the GraphAsMatrix class
	introduced in Program 
	to implement the Digraph interface
	defined in Program 
:
public class DigraphAsMatrix
    extends GraphAsMatrix
    implements Digraph
{
    // ...
}
	Implement the DigraphAsMatrix class
	by providing suitable definitions for the following methods:
	DigraphAsMatrix (constructor),
	purge,
	addEdge, getEdge, isEdge, and getEdges.
	You must also have a complete implementation of the base class
	GraphAsMatrix (see Project 
).
	Write a test program and test your implementation.