# Add a HTML 'child' to self
	#
	#     var ul = new HTMLTag("ul")
	#     ul.add(new HTMLTag("li"))
	#     assert ul.write_to_string    == "<ul><li></li></ul>"
	# returns `self` for fluent programming
	fun add(child: HTMLTag): HTMLTag
	do
		children.add(child)
		return self
	end