code: def _validate_index(self, proposal):
          "Check the range of each proposed index."
          if all(0 <= i < len(self._options_labels) for i in proposal.value):
              return proposal.value
          else:
              raise TraitError('Invalid selection: index out of bounds'f _validate_index(self, proposal):
          "Check the range of each proposed index."
          if all(0 <= i < len(self._options_labels) for i in proposal.value):
              return proposal.value
          else:
              raise TraitError('Invalid selection: index out of bounds')
docs: Check the range of each proposed index.