code: public function nestedform(HTTPRequest $r)
      {
          $req = new BetterButtonsNestedFormRequest($this, $this->owner, $this->owner->ItemEditForm());
  
          return $req->handleRequest($r, DataModel::inst());
      }
docs: Handles all custom action from DataObjects and hands them off to a sub-controller.
  e.g. /nestedform?action=myDataObjectAction
  
  @param  HTTPRequest $r
  @return BetterButtonsNestedFormRequest