# Each Category.RENDER_TYPE value has a subclass here.

    RENDER_TYPES = {

    }

    # Produce a subclass of _ProductsForm which we can alter the base_fields on

ProductAndQuantity = namedtuple("ProductAndQuantity", ["product", "quantity"])

@register.assignment_tag(takes_context=True)

def available_categories(context):

    ''' Returns all of the available product categories '''

    return rego.Category.objects.all()

@register.assignment_tag(takes_context=True)

def invoices(context):

    ''' Returns all of the invoices that this user has. '''

    return rego.Invoice.objects.filter(cart__user=context.request.user)

@register.assignment_tag(takes_context=True)

def items_pending(context):

    ''' Returns all of the items that this user has in their current cart,

    )

    return all_items

@register.assignment_tag(takes_context=True)

def items_purchased(context):

    ''' Returns all of the items that this user has purchased '''

    return products_form, discounts, handled

@transaction.atomic

def set_quantities_from_products_form(products_form, current_cart):

    for product_id, quantity, field_name in products_form.product_quantities():

        raise ValidationError("Cannot add that stuff")

    current_cart.end_batch()

def handle_voucher(request, prefix):

    ''' Handles a voucher form in the given request. Returns the voucher

    form instance, and whether the voucher code was handled. '''

    return (voucher_form, handled)

@login_required

def checkout(request):

    ''' Runs checkout for the current cart of items, ideally generating an