'Replace the given C{key}. It must already exist in the server.
@param key: the key to replace.
@type key: L{bytes}
@param val: the new value associated with the key.
@type val: L{bytes}
@param flags: the flags to store with the key.
@type flags: L{int}
@param expireTime: if different from 0, the relative time in seconds
when the key will be deleted from the store.
@type expireTime: L{int}
@return: a deferred that will fire with C{True} if the operation has
succeeded, and C{False} with the key didn\'t previously exist.
@rtype: L{Deferred}'

def replace(self, key, val, flags=0, expireTime=0):

    return self._set('replace', key, val, flags, expireTime, '')
