docs: def _insert_dummy_cart(self, exception, last_valid_cartesian=None):
          """Insert dummy atom into the already built cartesian of exception
          """
          def get_normal_vec(cartesian, reference_labels):
              b_pos, a_pos, d_pos = cartesian._get_positions(reference_labels)
              BA = a_pos - b_pos
              AD = d_pos - a_pos
              N1 = np.cross(BA, AD)
              n1 = N1 / np.linalg.norm(N1)
              return n1
  
          def insert_dummy(cartesian, reference_labels, n1):
              cartesian = cartesian.copy()
              b_pos, a_pos, d_pos = cartesian._get_positions(reference_labels)
              BA = a_pos - b_pos
              N2 = np.cross(n1, BA)
              n2 = N2 / np.linalg.norm(N2)
              i_dummy = max(self.index) + 1
              cartesian.loc[i_dummy, 'atom'] = 'X'
              cartesian.loc[i_dummy, ['x', 'y', 'z']] = a_pos + n2
              return cartesian, i_dummy
  
          if last_valid_cartesian is None:
              last_valid_cartesian = self._metadata['last_valid_cartesian']
          ref_labels = self.loc[exception.index, ['b', 'a', 'd']]
          n1 = get_normal_vec(last_valid_cartesian, ref_labels)
          return insert_dummy(exception.already_built_cartesian, ref_labels, n1)