# Look up an attribute's value by index.
	#
	# If the attribute value is a list of tokens (`IDREFS`,
	# `ENTITIES`, or `NMTOKENS`), the tokens will be concatenated
	# into a single string with each token separated by a
	# single space.
	#
	# Parameters:
	#
	# * `index: Int`: attribute index.
	# * `index: String`: XML 1.0 qualified (prefixed) name.
	# In many cases, it will be more efficient to look up the name once and
	# query by `Int` index rather than quering by name repeatedly.
	#
	# Returns:
	#
	# The attribute's value as a string, or `null` if the specified
	# attribute is not in the list or if qualified names
	# are not available.
	#
	# SEE: `length`
	redef fun value_of(index) do
		if index isa Int then
			if index >= 0 and index < length then
				return data[index * 5 + 4]
			end
		else if index isa String and "" != index then
			var i = 0

			while i < data.length do
				if data[i + 2] == index then
					return data[i + 4]
				end
				i += 5
			end
		end
		return null
	end