******** fig9.7 **********

	void
	topsort( graph G )
	{
		QUEUE Q;
		unsigned int counter;
		vertex v, w;

/*1*/		Q = create_queue( NUM_VERTEX ); make_null( Q ); counter = 0;
/*2*/		for each vertex v
/*3*/			if( indegree[v] == 0 )
/*4*/				enqueue( v, Q );

/*5*/		while( !is_empty( Q ) )
		{
/*6*/			v = dequeue( Q );
/*7*/			top_num[v] = ++counter;		/* assign next number */

/*8*/			for each w adjacent to v	
/*9*/				if( --indegree[w] == 0 )
/*10*/					enqueue( w, Q );
		}
/*11*/		if( counter != NUM_VERTEX )
/*12*/			error("Graph has a cycle");

/*13*/		dispose_queue( Q );		/* free the memory */
	}