def format(self,

               fmt: Optional[str]='#,#00.00 ¤¤',

               sep: str=', ',

               empty: str="Zero balance",

    ) -> str:

        """Formats the balance as a string with the given parameters

        If the balance is zero, returns ``empty``. Otherwise, returns a string

        with each amount in the balance formatted as ``fmt``, separated by

        ``sep``.

        If you set ``fmt`` to None, amounts will be formatted according to the

        user's locale. The default format is Beancount's input format.

        """

        amounts = [amount for amount in self.values() if amount.number]

        if not amounts:

            return empty

        amounts.sort(key=lambda amt: abs(amt.number), reverse=True)

        return sep.join(

            babel.numbers.format_currency(amt.number, amt.currency, fmt)

            for amt in amounts

        )

class MutableBalance(Balance):

    __slots__ = ()

    def add_amount(self, amount: data.Amount) -> None:

        try:

            self._currency_map[amount.currency] += amount.number

        except KeyError:

            self._currency_map[amount.currency] = amount.number

class RelatedPostings(Sequence[data.Posting]):

    """Collect and query related postings

    This class provides common functionality for collecting related postings

    and running queries on them: iterating over them, tallying their balance,

    etc.

    This class doesn't know anything about how the postings are related. That's

    entirely up to the caller.

    A common pattern is to use this class with collections.defaultdict

    to organize postings based on some key. See the group_by_meta classmethod

    for an example.

    """

    def __init__(self, source: Iterable[data.Posting]=()) -> None:

        self._postings: List[data.Posting] = list(source)

    @classmethod

    def group_by_meta(cls,

                      postings: Iterable[data.Posting],

                      key: MetaKey,

                      default: Optional[MetaValue]=None,

    ) -> Mapping[Optional[MetaValue], 'RelatedPostings']:

        """Relate postings by metadata value

        This method takes an iterable of postings and returns a mapping.

        The keys of the mapping are the values of post.meta.get(key, default).

        The values are RelatedPostings instances that contain all the postings

        that had that same metadata value.

        """

        retval: DefaultDict[Optional[MetaValue], 'RelatedPostings'] = collections.defaultdict(cls)

        for post in postings:

            retval[post.meta.get(key, default)].add(post)

        retval.default_factory = None

        return retval

    @overload

    def __getitem__(self, index: int) -> data.Posting: ...

    @overload

    def __getitem__(self, s: slice) -> Sequence[data.Posting]: ...

    def __getitem__(self,

                    index: Union[int, slice],

    ) -> Union[data.Posting, Sequence[data.Posting]]:

        if isinstance(index, slice):

            raise NotImplementedError("RelatedPostings[slice]")

        else:

            return self._postings[index]

    def __len__(self) -> int:

        return len(self._postings)

    def add(self, post: data.Posting) -> None:

        self._postings.append(post)

    def all_meta_links(self, key: MetaKey) -> Set[str]:

        retval: Set[str] = set()

        for post in self:

            try:

                retval.update(post.meta.get_links(key))