def back(self):
        if self._index <= 0:
            return None
        undone = self.current_item
        self._index -= 1
        self._check_index()
        return undone
QR factorization - Generate Q from QR factorization
dataset/ETHPy150Open kwikteam/phy/phy/cluster/manual/_history.py/History.back--crossover with--dataset/ETHPy150Open lebedov/scikit-cuda/skcuda/cula.py/culaDeviceCungqr