code: def dec(self, key, delta=1):
          
          self.set(key, (self.get(key) or 0) - delta)
docs: Decrements the value of a key by `delta`.  If the key does
          not yet exist it is initialized with `-delta`.
  
          For supporting caches this is an atomic operation.
  
          :param key: the key to increment.
          :param delta: the delta to subtract.