docs: function (keyName) {
        var keys = this._keys;
        if (!keys) {
          keys = this._keys = new _emberMetalEmpty_object.default();
        }
  
        if (!keys[keyName]) {
          keys[keyName] = 1;
          var content = this._content;
          var len = _emberMetalProperty_get.get(content, 'length');
  
          addObserverForContentKey(content, keyName, this, 0, len);
        } else {
          keys[keyName]++;
        }
      }