'Add delta to value in the cache. If the key does not exist, raise a
ValueError exception.'

def incr(self, key, delta=1, version=None):

    value = self.get(key, version=version)
    if (value is None):
        raise ValueError(("Key    '%s'    not    found" % key))
    new_value = (value + delta)
    self.set(key, new_value, version=version)
    return new_value
