`cart_name` - Name of this release cart
        `cart_description` - list of ['reponame', item1, ..., itemN] lists
        `manifests` - a list of manifest files

def update(self, cart_name, cart_description, manifests):
        """
        `cart_name` - Name of this release cart
        `cart_description` - list of ['reponame', item1, ..., itemN] lists
        `manifests` - a list of manifest files
        """
        if cart_description is None:
            juicer.utils.Log.log_debug("No cart_description provided.")
            cart_description = []

        if manifests is None:
            juicer.utils.Log.log_debug("No manifests provided.")
            manifests = []

        juicer.utils.Log.log_debug("Loading cart '%s'." % cart_name)
        cart = juicer.common.Cart.Cart(cart_name, autoload=True)

        for repo_items in cart_description:
            (repo, items) = (repo_items[0], repo_items[1:])
            juicer.utils.Log.log_debug("Processing %s input items for repo '%s'." % (len(items), repo))

            if repo not in cart.keys():
                cart[repo] = items
            else:
                for item in items:
                    cart[repo].append(juicer.common.CartItem.CartItem(os.path.expanduser(item)))

        for manifest in manifests:
            cart.add_from_manifest(manifest, self.connectors)

        cart.save()
        return cart