code: public synchronized ID replaceSessionId(final USER user, final ID oldSessionId,
  		final ID newSessionId, final SESSION newSession)
  	{
  		remove(oldSessionId);
  		return addOnline(user, newSessionId, newSession);
  	}
docs: Replace the given old session id with the new one.
  
  @param user
  the user
  @param oldSessionId
  the old session id
  @param newSessionId
  the new session id
  @param newSession
  the new session object
  @return the new session id that is associated with the given user.