code: def _add_label_edges(self):
          
          labels = self.ast.select('atom_label')
          if not labels:
              return
  
          # We need each individual label and atoms with multiple ring labels
          # would yield e.g. the string '12' so split those up.
          label_digits = defaultdict(list)
          for label in labels:
              digits = list(label.tail[0])
              for digit in digits:
                  label_digits[digit].append(label.parent())
  
          for label, (atom1, atom2) in label_digits.items():
              atom1_idx = self._atom_indices[id(atom1)]
              atom2_idx = self._atom_indices[id(atom2)]
              self.add_edge(atom1_idx, atom2_idx)
docs: Add edges between all atoms with the same atom_label in rings.