@abc.abstractmethod
    def __delitem__(self, key):
Compare-And-Set update.
dataset/ETHPy150Open peterhudec/authomatic/authomatic/extras/interfaces.py/BaseSession.__delitem__--crossover with--dataset/ETHPy150Open AppScale/appscale/AppServer/google/appengine/api/memcache/__init__.py/Client.cas