docs: def invert(self):
          '''
          Invert by swapping each value with its key.
  
          Returns
          -------
          MultiDict
              Inverted multi-dict.
  
          Examples
          --------
          >>> MultiDict({1: {1}, 2: {1,2,3}}, 4: {}).invert()
          MultiDict({1: {1,2}, 2: {2}, 3: {2}})
          '''
          result = defaultdict(set)
          for k, val in self.items():
              result[val].add(k)
          return MultiDict(dict(result))