code: def _second_column(self):
          
  
          if self._A[1, 1] == 0 and self._A[2, 1] != 0:
              self._swap_rows(1, 2)
  
          if self._A[2, 1] != 0:
              self._zero_second_column()
docs: Right-low 2x2 matrix
  
          Assume elements in first row and column are all zero except for A[0,0].