<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def PATTERN(self):
 u""" Uses the mapping of names to features to return a PATTERN suitable for using the lib2to3 patcomp. """

        self.update_mapping()
        return u" |\n".join([pattern_unformatted % (f.name, f._pattern) for f in iter(self)])