# Returns true if and only if `u` exists in this graph.
	#
	# ~~~
	# var g = new HashDigraph[Int]
	# g.add_vertex(1)
	# assert g.has_vertex(1)
	# assert not g.has_vertex(0)
	# g.add_vertex(1)
	# assert g.has_vertex(1)
	# assert not g.has_vertex(0)
	# ~~~
	fun has_vertex(u: V): Bool is abstract