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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 _ref_check(self, case):
 """ Checks that there is only one reference bus. """

        refs = [bus._i for bus in case.buses if bus.type == REFERENCE]

        if len(refs) == 1:
            return True, refs
        else:
            logger.error("OPF requires a single reference bus.")
            return False, refs