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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 _set_schedules(self):
 """ Attach the known and unknown schedules """

        self.schedules = ['ReturnHeader990x', ]
        self.otherforms = []
        for sked in self.raw_irs_dict['Return']['ReturnData'].keys():
            if not sked.startswith("@"):
                if sked in KNOWN_SCHEDULES:
                    self.schedules.append(sked)
                else:
                    self.otherforms.append(sked)