docs: def to_index(self):
          """Convert this variable to a pandas.Index"""
          # n.b. creating a new pandas.Index from an old pandas.Index is
          # basically free as pandas.Index objects are immutable
          assert self.ndim == 1
          index = self._data.array
          if isinstance(index, pd.MultiIndex):
              # set default names for multi-index unnamed levels so that
              # we can safely rename dimension / coordinate later
              valid_level_names = [name or '{}_level_{}'.format(self.dims[0], i)
                                   for i, name in enumerate(index.names)]
              index = index.set_names(valid_level_names)
          else:
              index = index.set_names(self.name)
          return index