uuid = function b(a) {
  return a ? (a ^ Math.random() * 16 >> a / 4).toString(16) :
      ([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, b);
}
Generates a UUID.
https://gist.github.com/jed/982883
@param {string|undefined=} a
@return {string}