code: public function setSequential( $sequential )
      {
          if (preg_match( '/^[1-9]$/', $sequential ) !== 1) {
              throw new InvalidArgumentException( sprintf( 'Sequential number should be between 1 and 9 (%s)',
                  $sequential ) );
          }
  
          $this->sequential = intval( $sequential );
  
          return $this;
      }
docs: @param  int $sequential
  
  @return $this