docs: def assert_unique_multiindex_level_names(variables):
      """Check for uniqueness of MultiIndex level names in all given
      variables.
  
      Not public API. Used for checking consistency of DataArray and Dataset
      objects.
      """
      level_names = defaultdict(list)
      all_level_names = set()
      for var_name, var in variables.items():
          if isinstance(var._data, PandasIndexAdapter):
              idx_level_names = var.to_index_variable().level_names
              if idx_level_names is not None:
                  for n in idx_level_names:
                      level_names[n].append('%r (%s)' % (n, var_name))
              if idx_level_names:
                  all_level_names.update(idx_level_names)
  
      for k, v in level_names.items():
          if k in variables:
              v.append('(%s)' % k)
  
      duplicate_names = [v for v in level_names.values() if len(v) > 1]
      if duplicate_names:
          conflict_str = '\n'.join([', '.join(v) for v in duplicate_names])
          raise ValueError('conflicting MultiIndex level name(s):\n%s'
                           % conflict_str)
      # Check confliction between level names and dimensions GH:2299
      for k, v in variables.items():
          for d in v.dims:
              if d in all_level_names:
                  raise ValueError('conflicting level / dimension names. {} '
                                   'already exists as a level name.'.format(d))