******** fig9.5 **********

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

		for(  counter=0; counter < NUM_VERTEX; counter++ )
		{
			v = find_new_vertex_of_indegree_zero;
			if( v == NOT_A_VERTEX )
			{
				error("Graph has a cycle");
				break;
			}
			top_num[v] = counter;
			for each w adjacent to v	
				indegree[w]--;
		}
	}