code: def _second_finalize(self):
          
  
          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)
docs: Set zero at A[2, 1]
  
          This is possible only when A[2,1] is dividable by A[1,1].