function uuid() {
    // RFC4122 http://www.ietf.org/rfc/rfc4122.txt
    var d = Date.now()
    return 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, function(c) {
        var r = (d + Math.random()*16)%16 | 0
        d = Math.floor(d/16)
        return (c=='x' ? r : (r&0x3|0x8)).toString(16)
    })
};