# Adds all vertices of `vertices` to this digraph.
	#
	# If vertices appear more than once, they are only added once.
	#
	# ~~~
	# var g = new HashDigraph[Int]
	# g.add_vertices([0,1,2,3])
	# assert g.num_vertices == 4
	# g.add_vertices([2,3,4,5])
	# assert g.num_vertices == 6
	# ~~~
	fun add_vertices(vertices: Collection[V])
	do
		for u in vertices do add_vertex(u)
	end