code: def update(self, cart_name, cart_description, manifests):
          
          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
docs: `cart_name` - Name of this release cart
          `cart_description` - list of ['reponame', item1, ..., itemN] lists
          `manifests` - a list of manifest files