I've just chanced upon this and thought it might be of interest to some of you, haven't tried it myself http://www.grantjenks.com/docs/sortedcontainers/ -- My fellow Pythonistas, ask not what our language can do for you, ask what you can do for our language. Mark Lawrence --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com