# Create a new HTMLTag child and return it
	#
	#     var ul = new HTMLTag("ul")
	#     ul.open("li").append("1").append("2")
	#     ul.open("li").append("3").append("4")
	#     assert ul.write_to_string    == "<ul><li>12</li><li>34</li></ul>"
	fun open(tag: String): HTMLTag
	do
		var res = new HTMLTag(tag)
		add(res)
		return res
	end