# Content of this XML tag held within a `PCDATA` or `CDATA`
	#
	# ~~~
	# var code = """
	# <?xml version="1.0" encoding="us-ascii"?>
	# <animal>
	#     <cat/>
	#     <tiger>This is a white tiger!</tiger>
	#     <cat/>
	# </animal>"""
	#
	# var xml = code.to_xml
	# assert xml["animal"].first["tiger"].first.as(XMLStartTag).data == "This is a white tiger!"
	# ~~~
	fun data: nullable String
	do
		for child in children do
			if child isa PCDATA then return child.content
			if child isa CDATA then return child.content
		end
		return null
	end