code: def convert(self, src: type, tar: type) -> Callable[[T], B]:
          
          return self.converters[(src, tar)]
docs: doufo.ConvertDict().convert: define a converter from `type src` to `type tar`   
              Attibutes:   
                  `self`
                  `src` (`type`): source `type`,    
                  `tar` (`type`): target `type`,    
              Returns:
                  `converter` (`Callable[[T], B]`): converter from `type src` to `type tar`