# Look up an attribute's Namespace URI by index.
	#
	# Parameters:
	#
	# * `index`: attribute index.
	#
	# Returns:
	#
	# The Namespace URI, or the empty string if none
	# is available, or `null` if the index is out of
	# range.
	#
	# SEE: `length`
	fun uri(index: Int): nullable String is abstract