(define (insert-products products tree)
  (local(
         (define (insert-prod prod _tree)
           (cond [(empty? _tree) (make-product-tree-node empty empty prod)]
              [else (cond
                 [(< (product-id prod) (product-id (product-tree-node-prod _tree)))
                  (make-product-tree-node (insert-prod prod (product-tree-node-left _tree)) (product-tree-node-right _tree) (product-tree-node-prod _tree))]
                 [(> (product-id prod) (product-id (product-tree-node-prod _tree)))
                  (make-product-tree-node (product-tree-node-left _tree) (insert-prod prod (product-tree-node-right _tree)) (product-tree-node-prod _tree))]
                 [(= (product-id prod) (product-id (product-tree-node-prod _tree))) (insert-products (rest products) tree)]
                 )])))
    (cond
      [(empty? products) tree]
      [else (insert-products (rest products) (insert-prod (first products) tree))])))