def nick_changed(self,user,old_nick,stanza):
pass<EOL>
Called after a user nick has been changed.

:Parameters:
    - `user`: the user (after update).
    - `old_nick`: the old nick.
    - `stanza`: the stanza received.

:Types:
    - `user`: `MucRoomUser`
    - `old_nick`: `unicode`
    - `stanza`: `pyxmpp.stanza.Stanza`