<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def copies(self):
 """ All test case copies created by the user """

        import nitrate
        if self._copies is None:
            log.info(u"Searching for cases copied by {0}".format(self.user))
            self._copies = [
                case for case in nitrate.TestCase.search(
                    author__email=self.user.email,
                    create_date__gt=str(self.options.since),
                    create_date__lt=str(self.options.until),
                    tag__name=TEST_CASE_COPY_TAG)]
        return self._copies