*/        
// Add all linkages here       
// Some examples are provided:       
 
     /*
                $this->add_prod_to_prod(3, 83, 'X', 0);
                $this->add_prod_to_prod(3, 3, "%", 100);
                $this->add_prod_to_prod(3, 83, "%", 100);
                $this->add_prod_to_prod(3, 27, "%", 100);
                $this->add_cat_to_prod(4, 3, "%", 100);
                $this->add_cat_to_prod(4, 83, "%", 100);
                // Buy product 83, get product 53 at 50% off
                $this->add_prod_to_prod(83, 53, "%", 50);

                 // Buy product 83, get one free
                $this->add_prod_to_prod(83, 83, "%", 100);

                 // Buy product 83, get an item from category 14 free
                $this->add_prod_to_cat(83, 14, "%", 100);

                 // Buy an item from category 21, get an item from category 14 free
                $this->add_cat_to_cat(21, 14, "%", 100);

                 // Buy item 12, get a second one free. 
               $this->add_twoforone_prod(12);

                 // Buy any item from category 10, get a second identical one free
                $this->add_twoforone_cat(10);

                 // $this->add_twoforone_prod(17);
                $this->add_prod_to_prod(26, 27, "%", 100);
                $this->add_prod_to_prod(83, 15, "%", 50);
                $this->add_prod_to_prod(83, 20, "%", 25);
                $this->add_cat_to_cat(14, 14, "%", 100);
                $this->add_prod_to_prod(3, 25, "%", 50);
       */
    }
  }