code: function create(callback) {
    /* jslint validthis: true */
    for (var
      queue = this,
      args = [],
      info = {
        id: {},
        fn: callback,
        ar: args
      },
      i = 1; i < arguments.length; i++
    ) args[i - 1] = arguments[i];
    return infoId(queue, info) || (queue.push(info), info.id);
  }
docs: create a unique id and returns it if the callback with same extra arguments was already scheduled, then returns same id