# Add an edge between all elements of `es` in order.
	#
	# ~~~~
	# var pos = new POSet[String]
	# pos.add_chain(["A", "B", "C", "D"])
	# assert pos.has_direct_edge("A", "B")
	# assert pos.has_direct_edge("B", "C")
	# assert pos.has_direct_edge("C", "D")
	# ~~~~
	fun add_chain(es: SequenceRead[E])
	do
		if es.is_empty then return
		var i = es.iterator
		var e = i.item
		i.next
		for f in i do
			add_edge(e, f)
			e = f
		end
	end