def get_fs(self, key):
        val = self[key]
        if not isinstance(val, FeatureStructure):
            raise ValueError('Value for key %r is not a FeatureStructure' % key)
        return val
Test an erroneous add: if a L{MemCacheProtocol.add} is called but the
dataset/ETHPy150Open cidles/graf-python/src/graf/annotations.py/FeatureStructure.get_fs--crossover with--dataset/ETHPy150Open kuri65536/python-for-android/python-modules/twisted/twisted/test/test_memcache.py/CommandMixin.test_errorAdd