docs: def fmap(self, f: Callable[[T], B]) -> 'Maybe[B]':
          """doufo.maybe.fmap: map a `Maybe` object with a function  
          Args:  
              `self`  
              f (`Callable[[T], B]`): a function    
          Returns:  
              return (`Maybe[B]`): returned `Maybe` objects from function `f`    
          Raises:
          """
          return Maybe(None) if self.unbox() is None else Maybe(f(self.unbox()))