docs: def register(self, src: type, tar: type) -> Callable[[T], B]:
          """doufo.ConverterDict().register(): A decorator factory to define typing converting decorator
              Attributes:  
                  `self`
                  `src` (`type`): source `type`,
                  `tar` (`type`): target `type`,
              Returns:
                  `f` (`Callable[[T], B]`): a decorater that defines a converter                
          """
          def deco(f):
              self.converters[(src, tar)] = f
              self.converters = self.sorted_converters_keys()
              return f
          return deco