docs: function (content, idx, removedCnt, addedCnt) {
        var keys = this._keys;
        var lim = removedCnt > 0 ? idx + removedCnt : -1;
        for (var key in keys) {
          if (lim > 0) {
            removeObserverForContentKey(content, key, this, idx, lim);
          }
          _emberMetalProperty_events.propertyWillChange(this, key);
        }
      }