# Returns true if and only if `vertices` is a circuit of this digraph.
	#
	# ~~~
	# var g = new HashDigraph[Int]
	# g.add_arc(1, 2)
	# g.add_arc(2, 3)
	# g.add_arc(3, 1)
	# assert g.has_circuit([1,2,3,1])
	# assert not g.has_circuit([1,3,2,1])
	# ~~~
	fun has_circuit(vertices: SequenceRead[V]): Bool
	do
		return vertices.is_empty or (has_path(vertices) and vertices.first == vertices.last)
	end