# Returns the outgoing arcs of vertex `u`.
	#
	# If `u` is not in this graph, an empty array is returned.
	#
	# ~~~
	# var g = new HashDigraph[Int]
	# g.add_arc(1, 3)
	# g.add_arc(2, 3)
	# g.add_arc(1, 2)
	# for arc in g.outgoing_arcs(1) do
	#	assert g.is_successor(arc[1], arc[0])
	# end
	# ~~~
	fun outgoing_arcs(u: V): Collection[Array[V]]
	do
		if has_vertex(u) then
			return [for v in successors(u) do [u, v]]
		else
			return new Array[Array[V]]
		end
	end