code: def swap(self):
          # type: () -> DictWrapper
          
          return self.__class__((v, k) for k, v in self.items())
docs: Swap key and value
  
          /!\ Be carreful, if there are duplicate values, only one will
          survive /!\
  
          Example:
  
              >>> from ww import d
              >>> d({1: 2, 2: 2, 3: 3}).swap()
              {2: 2, 3: 3}