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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 _gen_back(self):
        '''
        Return a list of loaded roster backends
        '''
        back = set()
        if self.backends:
            for backend in self.backends:
                fun = '{0}.targets'.format(backend)
                if fun in self.rosters:
                    back.add(backend)
            return back
        return sorted(back)