# Adds all arcs of `arcs` to this digraph.
	#
	# If arcs appear more than once, they are only added once.
	#
	# ~~~
	# var g = new HashDigraph[Int]
	# var arcs = [[0,1], [1,2], [1,2]]
	# g.add_arcs(arcs)
	# assert g.num_arcs == 2
	# ~~~
	fun add_arcs(arcs: Collection[Array[V]])
	do
		for a in arcs do add_arc(a[0], a[1])
	end