# Returns the all successors of `u`.
	#
	# `u` is include in the returned collection
	#
	# Returns an empty Array is the `u` does not exist
	# ~~~
	# var g = new HashDigraph[Int]
	# g.add_arc(1, 2)
	# g.add_arc(2, 3)
	# g.add_arc(3, 4)
	# assert g.get_all_successors(2).has(3)
	# assert g.get_all_successors(2).has(4)
	# assert g.get_all_successors(2).has(2)
	# ~~~
	fun get_all_successors(u: V): Array[V]
	do
		if not vertices.has(u) then return new Array[V]
		if not cache_all_successors.has_key(u) then compute_all_link(u)
		return cache_all_successors[u].clone.to_a
	end