docs: function subscribe(callback) {
      var id = store._getCurrentCallbackId();
      store._incrementCurFunctionId();
      function unsubscribe() {
        store._callback_objs = store._callback_objs.filter(function (c) {
          return c.id !== id;
        });
        store._removeKeySubscriber(id);
      }
      store._callback_objs.push({ id: id, callback: callback });
      return unsubscribe;
    }