# Returns an iterator over the arcs of this graph
	#
	# ~~~
	# var g = new HashDigraph[Int]
	# g.add_arc(0, 1)
	# g.add_arc(0, 2)
	# g.add_arc(1, 2)
	# for arc in g.arcs_iterator do
	#	assert g.has_arc(arc[0], arc[1])
	# end
	# ~~~
	fun arcs_iterator: Iterator[Array[V]] do return new ArcsIterator[V](self)