code: function (state, index) {
                  var items = this._getItems(state.choice);
                  if (index < items.length) {
                      return items[index];
                  }
                  return null;
              }
docs: Modify state to move to (and get) the next item (if any)
  
  @param {Object} state
  @param {Number} index
  @return {PatternItem}
  @private