my.uuid4 = function() {
    return 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, my._uuid4);
  };
  //// OPTIMIZATION - cache callback
  my._uuid4 = function(cc) {
    var rr = Math.random() * 16 | 0; return (cc === 'x' ? rr : (rr & 0x3 | 0x8)).toString(16);
  };