docs: function (optionList, result, parentOption) {
          zrUtil.each(optionList, function (option) {
              if (!option) {
                  return;
              }
  
              if (parentOption) {
                  option.parentOption = parentOption;
              }
  
              result.push(option);
  
              var children = option.children;
              if (option.type === 'group' && children) {
                  this._flatten(children, result, option);
              }
              // Deleting for JSON output, and for not affecting group creation.
              delete option.children;
          }, this);
      }