Set zero at A[2, 1]

        This is possible only when A[2,1] is dividable by A[1,1].

def _second_finalize(self):
        """Set zero at A[2, 1]

        This is possible only when A[2,1] is dividable by A[1,1].

        """

        A = self._A
        L = np.eye(3, dtype='intc')
        L[2, 1] = -A[2, 1] // A[1, 1]
        self._L.append(L.copy())
        self._A = np.dot(L, self._A)