code: function (context) {
              context = context || {};
  
              var layerCounter,
                  layers = this._layers,
                  layer,
                  data = this.data;
  
              var keySets = [_.difference(_.keys(data), this._exclusions)];
              for (layerCounter = 0; layerCounter < layers.length; layerCounter++) {
                  layer = layers[layerCounter];
                  keySets.push(layer.getKeys(data[layer.key], context));
              }
  
              return _.union.apply(null, keySets);
          }
docs: Get the preference IDs that are set in this Scope. All layers are added
  in. If context is not provided, the set of all keys in the Scope including
  all keys in each layer will be returned.
  
  @param {?Object} context Optional additional information for looking up the keys
  @return {Array.<string>} Set of preferences set by this Scope