docs: def incr(self, key, delta=1):
          """
          Add delta to value in the cache. If the key does not exist, raise a
          ValueError exception.
          """
          value = self.get(key)
          if value is None:
              raise ValueError("Key '%s' not found" % key)
          new_value = value + delta
          self.set(key, new_value)
          return new_value