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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def tasks(self):
 """ All completed tasks within specified time range """

        if self._tasks is None:
            self._tasks = self.parent.tasks.tasks(
                tasklist="@default", showCompleted="true", showHidden="true",
                completedMin=self.since, completedMax=self.until)
        log.info(u"NB TASKS {0}".format(len(self._tasks)))
        return self._tasks