# Returns true if and only if `u` is a successor of `v`.
	#
	# ~~~
	# var g = new HashDigraph[Int]
	# g.add_arc(1, 3)
	# assert not g.is_successor(1, 3)
	# assert g.is_successor(3, 1)
	# ~~~
	fun is_successor(u, v: V): Bool do return has_arc(v, u)