# Returns the number of arcs whose target is `u`.
	#
	# ~~~
	# var g = new HashDigraph[Int]
	# g.add_arc(1, 3)
	# g.add_arc(2, 3)
	# assert g.in_degree(3) == 2
	# assert g.in_degree(1) == 0
	# ~~~
	fun in_degree(u: V): Int do return predecessors(u).length