/**
  * Reduce stock level of the product
  * Acts as an alias for modify_stock()
  *
  * @param   int   Amount to reduce by
  * @return  int
  */
 public function reduce_stock($by = -1)
 {
     if ($this->stock == '-9999999') {
         $_parent = new fflcommerce_product($this->ID);
         return $_parent->modify_stock(-$by);
     } else {
         return $this->modify_stock(-$by);
     }
 }