code: public function exec($dateOn)
      {
          $prevCalcId = $this->getPrevTreeCalcId($dateOn);
          $tree = $this->daoBonDwnl->getByCalcId($prevCalcId);
          $result = $this->hlpTree->mapById($tree, EBonDwnl::A_CUST_REF);
          return $result;
      }
docs: Get plain tree for previous period (with unq. months data).
  
  @param string $dateOn YYYYMMDD
  @return EBonDwnl[]