# Returns a shortest path from vertex `u` to `v`.
	#
	# If no path exists between `u` and `v`, it returns `null`.
	#
	# ~~~
	# var g = new HashDigraph[Int]
	# g.add_arc(1, 2)
	# g.add_arc(2, 3)
	# g.add_arc(3, 4)
	# assert g.a_shortest_path(1, 4).length == 4
	# g.add_arc(1, 3)
	# assert g.a_shortest_path(1, 4).length == 3
	# assert g.a_shortest_path(4, 1) == null
	# ~~~
	fun a_shortest_path(u, v: V): nullable Sequence[V]
	do
		var queue = new List[V].from([u]).as_fifo
		var pred = new HashMap[V, nullable V]
		var visited = new HashSet[V]
		var w: nullable V = null
		pred[u] = null
		while not queue.is_empty do
			w = queue.take
			if not visited.has(w) then
				visited.add(w)
				if w == v then break
				for wp in successors(w) do
					if not pred.keys.has(wp) then
						queue.add(wp)
						pred[wp] = w
					end
				end
			end
		end
		if w != v then
			return null
		else
			var path = new List[V]
			path.add(v)
			w = v
			while pred[w] != null do
				path.unshift(pred[w].as(not null))
				w = pred[w]
			end
			return path
		end
	end