# Display an error
	#
	# Return the message (to add information)
	fun error(l: nullable Location, s: String): Message
	do
		var m = new Message(l, null, s, 2)
		if messages.has(m) then return m
		if l != null then l.add_message m
		if opt_warn.value <= -1 then return m
		messages.add m
		error_count = error_count + 1
		if opt_stop_on_first_error.value then check_errors
		return m
	end