<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def lookup_string(self, keysym):
 """Return a string corresponding to KEYSYM, or None if no reasonable translation is found. """

        s = self.keysym_translations.get(keysym)
        if s is not None:
            return s

        import Xlib.XK
        return Xlib.XK.keysym_to_string(keysym)