'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.
:returns: The new value or `None` for backend errors.'

def dec(self, key, delta=1):

    value = ((self.get(key) or 0) - delta)
    return (value if self.set(key, value) else None)
