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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 all_low_level_calls(self):
 """ recursive version of low_level calls """

        if self._all_low_level_calls is None:
            self._all_low_level_calls = self._explore_functions(lambda x: x.low_level_calls)
        return self._all_low_level_calls