'Allow simple extension commands
notified by server in CAPABILITY response.
Assumes command is legal in current state.
(typ, [data]) = <instance>.xatom(name, arg, ...)
Returns response appropriate to extension command `name\'.'

def xatom(self, name, *args):

    name = name.upper()
    if (name not in Commands):
        Commands[name] = (self.state,)
    return self._simple_command(name, *args)
