code: def bind(self, f: Callable[[T], 'Maybe[B]']) -> 'Maybe[B]':
          
          return self.fmap(lambda x: f(x).unbox())
docs: doufo.Maybe.bind  
          Args:  
              `self`:  
              f (`Callable[[T], Maybe[B]`): A function that returns `Maybe[B]`  
          Returns:  
              return (`Maybe[B]`): results of mapping in form of `Maybe[B]`  
          Raises: