code: public function startAt($startAt=0){
          if (!is_numeric($startAt) || $startAt < 0){
              throw new Exception("startAt: bad value", 10);
          }
          $this->startAt = $startAt;
          return $this;
      }
docs: Start at the line for Add
  
  @param int $startAt
  
  @return $this
  @throws Exception