'Adds delta to the cache version for the supplied key. Returns the
new version.'

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

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