\startluacode
	-- Define our namespace as hvdm
	hvdm = hvdm or {}

-- Function checks the correctness of an XML tree.
	hvdm.checkXMLtree = function (xml, linesep)

	local xml = xml and xml or ""
	local linesep = linesep and linesep or "\n"
	local badspace = {}

	-- Empty files do not need processing.
	if xml == "" then
		return ""
	end

	-- Step one is remove leading characters before first node.
	local leadingnode = string.find(xml, "<", 1) or 1
	if leadingnode ~= 1 then
		xml = string.sub(xml, leadingnode, -1)
	end

	-- Strip all comments <!-- --> but node pairs inside make it complicated.
	local c1, c2 = string.find(xml, "<!--", 1, true)
	while c1 do
		-- Search for closing of comment.
		local c3, c4 = string.find(xml, "-->", c2 + 1, true)
		if c3 then
			-- Remove comment.
			xml = string.sub(xml, 1, c1 - 1) .. string.sub(xml, c4 + 1, -1)

			-- Search for next comment.
			c1, c2 = string.find(xml, "<!--", 1, true)
		else
			badspace[#badspace + 1] = " >>> Open comment <!-- Fix this first"
			c1 = nil
		end
	end

	-- Strip <?xml version ... ?> and DOCTYPE nodes.
	if string.find(xml, "<%?xml.*%?>") then
		xml = string.gsub(xml, "<%?xml.*%?>", "")
	else
		badspace[#badspace + 1] =
			" >>> missing or incorrect <?xml ?>"
	end
	xml = string.gsub(xml, "<.*DOCTYPE[^>]*>", "")

	-- Strip everything between > and <.
	xml = string.gsub(xml, ">[^<]*<", "><")

	-- Spot all cases  of '< node', etc. which have bad spaces.
	for node in string.gmatch(xml, "</?%s+[^>]*") do
		badspace[#badspace + 1] = " >>> erroneous whitespace in " .. node .. ">"
	end
	for node in string.gmatch(xml, "<[^>]*%s+/?>") do
		badspace[#badspace + 1] = " >>> erroneous whitespace in " .. node
	end

	-- Remove the bad spaces.
	if #badspace > 0 then
		xml = string.gsub(xml, "<%s+", "<")
		xml = string.gsub(xml, "</%s+", "</")
		xml = string.gsub(xml, "%s+>", ">")
		xml = string.gsub(xml, "%s+/>", "/>")
	end

	-- Strip all <xx/> nodes, must be done before stripping attributes.
	xml = string.gsub(xml, "<[^>]*/>", "")

	-- Strip attributes.
	local fa1, fa2, fa3 = 0, 0, 0
	while fa3 do
		local fa1,fa2 = string.find(xml, "<%S+%s+", 1)
		if fa1 then
			_, fa3 = string.find(xml, ">", fa2, true)
			if fa3 then
				xml = string.sub(xml, 1, fa2-1) .. string.sub(xml, fa3, -1)
			end
		end
	end

	-- Strip all remaining whitespace.
	xml = string.gsub(xml, "%s", "")

	local function nextnode(str, last)
		local newlast
		if last ~= nil then
			last, newlast = string.find(str, "</?[^>]*>", last + 1)
		end
		return newlast
	end

	-- At this point we have the bare <node> tree.
	-- Strip all adjacent pairs <node></node> recursively.
	-- The xml is correct when nothing is left.
	local function nextnode(str, last)
		local newlast
		if last ~= nil then
			last, newlast = string.find(str, "</?[^>]*>", last)
		end
		return newlast
	end

	-- Remove adjacent pairs <node></node>.
	local i1, i2, j1, j2
	i1, i2 = 1, nextnode(xml, 1)
	if i2 ~= nil then
		j1, j2 = i2 + 1, nextnode(xml, i2 + 1)
	end

	-- Loop until nothing left for a correct xml tree.
	while j1 ~= nil do

		-- Check if we have node> and /node>.
		if string.sub(xml, i1 + 1, i2) == string.sub(xml, j1 + 2, j2) then

			-- Here is a matching pair, remove all of them.
			local removal = string.sub(xml, i1, i2) .. string.sub(xml, j1, j2)
			xml = string.gsub(xml, removal, "")

			-- Start over at the beginning.
			i1, i2 = 1, nextnode(xml, 1)
			if i2 ~= nil then
				j1, j2 = i2 + 1, nextnode(xml, i2 + 1)
			else
				j1 = nil
			end
		else
			-- Shift to next pair of nodes.
			if j2 ~= nil then
				i1, i2, j1, j2 = j1, j2, j2 + 1, nextnode(xml, j2 + 1)
			else
				j1 = nil
			end
		end
	end

	-- If something is leftover then the xml tree is not correct.
	if #xml > 0 then
		badspace[#badspace + 1] = " >>> " .. xml
	end

	-- Note that xml might end up as a single newline chracter.
	if #badspace > 0 then
		return "XML tree is not correct" .. linesep ..
					table.concat(badspace, linesep) .. linesep
	end
	return ""

end	-- end of checkXMLtree

-- Function checks the correctness of an XML tree in given file.
	hvdm.checkXMLfile = function (filename)

	-- Filename must have suffix .xml.
	if hvdm.filesuffix(filename) ~= "xml" then
		return "Filename of " .. filename .. " missing suffix .xml"
	end

	-- Open the file for reading and check.
	local inputfile = io.open(filename, "r")
	if inputfile then
		-- Read contents into a string then close the file.
		local xml = inputfile:read("a")
		io.close(inputfile)

		-- Execute the test.
		local outcome = hvdm.checkXMLtree(xml, "\\crlf ")
		return outcome == "" and ""
						or  "XML check of file " .. filename .. " : " .. outcome
		-- Note that an empty tree has no errors.
	end
end	-- end of checkXMLfile

-- Function checks the correctness of an XML tree in given directory.
	hvdm.checkXMLfiles = function (directoryname)

	-- Make temp file for directory listing and open for writing.
	local tempname = "/tmp/hvdm_list_tmp.txt"
	local temp = io.open(tempname,"w+")

	-- Write diretory listingof .xml files to the temp file.
	os.execute("ls -1 " .. directoryname .. "/*" .. ".xml" .. " >" .. tempname)

	-- Reset temp file for reading of the directory entries.
	temp:seek("set")

	-- Retrieve line by line the filenames and put into a table.
	local filenames = {}
	local filename = temp:read("l")
	while filename ~= nil do
		filenames[#filenames + 1] = filename
		filename = temp:read("l")
	end

	-- Close the tempfile and remove it and its name.
	temp:close()
	os.remove(tempname)

	-- Initialize combined result.
	local outcome = ""

	-- Check one by one the xml files from the table.
	for _,name in ipairs(filenames) do
		outcome = outcome .. hvdm.checkXMLfile(name)
	end

	-- Return combined outcome.
	return outcome

end	-- end of checkXMLfiles

\stopluacode