# Returns an iterator over the vertices of this graph.
	#
	# ~~~
	# var g = new HashDigraph[Int]
	# g.add_arc(0, 1)
	# g.add_arc(0, 2)
	# g.add_arc(1, 2)
	# var vs = new HashSet[Int]
	# for v in g.vertices_iterator do vs.add(v)
	# assert vs == new HashSet[Int].from([0,1,2])
	# ~~~
	fun vertices_iterator: Iterator[V] is abstract