"You may only choose each product type once.",

                    )

                products.add(product)

                yield product, quantity

        for product in (all_products - products):

            yield product, 0

    def add_product_error(self, product, error):

        for form in self.forms:

            form.add_product_error(product, error)

class VoucherForm(forms.Form):

    voucher = forms.CharField(

        label="Voucher code",

        help_text="If you have a voucher code, enter it here",

        required=False,

    )

def staff_products_form_factory(user):

    ''' Creates a StaffProductsForm that restricts the available products to

    those that are available to a user. '''