# Returns the distance between `u` and `v`
	#
	# If no path exists between `u` and `v`, it returns null. It is not
	# symmetric, i.e. we may have `dist(u, v) != dist(v, u)`.
	#
	# ~~~
	# var g = new HashDigraph[Int]
	# g.add_arc(1, 2)
	# g.add_arc(2, 3)
	# g.add_arc(3, 4)
	# assert g.distance(1, 4) == 3
	# g.add_arc(1, 3)
	# assert g.distance(1, 4) == 2
	# assert g.distance(4, 1) == null
	# ~~~
	fun distance(u, v: V): nullable Int
	do
		var queue = new List[V].from([u]).as_fifo
		var dist = new HashMap[V, Int]
		var visited = new HashSet[V]
		var w: nullable V
		dist[u] = 0
		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 dist.keys.has(wp) then
						queue.add(wp)
						dist[wp] = dist[w] + 1
					end
				end
			end
		end
		return dist.get_or_null(v)
	end