ChangeCustomerProductsHandler

    /**
     * Handle command
     *
     * @param ChangeCustomerProducts $command
     */
    public function __invoke(ChangeCustomerProducts $command)
    {
        $customerId = $command->customerId();

        /** @var Customer $customer */
        if (!$customer = $this->customerRepository->get($customerId)) {
            throw CustomerNotFound::withCustomerId($customerId);
        }

        $removeFromPractice = function(PracticeId $practiceId, Set $productRemovals) use (PracticeRepository $practiceRepository) {
            $practice = $practiceRepository->get($practiceId);

            $practice->removeProducts($productRemovals);
        }
        
        $customer->changeProducts($command->requestedAdditions(), $command->requestedRemovals(), $removeFromPractice);
    }

ProductAggregate

    /**
     * @param Set $requestedAdditions
     * @param Set $requestedRemovals
     * @param callback $removeFromPractice
     */
    public function changeProducts(Set $requestedAdditions, Set $requestedRemovals, callback $removeFromPractice)
    {
        $additions = $this->products->union($requestedAdditions)->diff($this->products);
        $removals  = $this->products->xor($this->products->diff($requestedRemovals));

        if (!$additions->isEmpty() || !$removals->isEmpty()) {
            $this->recordThat(CustomerProductsChanged::withChanges($this->customerId, $additions, $removals));
        }
        
        foreach($this->practices as $practiceId) {
            $removeFromPractice($removeFromPractice, $removals);
        }
    }