code: protected function compute($n)
      {
          $int_fact = 1;
  
          for ($i = 1; $i <= $n ; $i++) {
              $int_fact *= $i;
          }
  
          return $int_fact;
      }
docs: Computes the factorial and returns the result internally.
  
  @param  integer $n The rank
  @access protected
  @return void