PostGroups = Mapping[Optional[str], 'AccrualPostings']

    def make_consistent(self) -> Iterator[Tuple[str, 'AccrualPostings']]:

            # mypy loses track of the fact that self.invoice must be a str in

            # this case (gated on the invoice_ok boolean) and complains it

            # doesn't match the type of the return value.

            yield (self.invoice, self)  # type:ignore[misc]

    # groups is a mapping of metadata value strings to AccrualPostings.

    # The keys are basically arbitrary, the report classes don't rely on them,

    # but they do help symbolize what's being grouped.

    # For the outgoing approval report, groups maps rt-id link strings to

    # associated accruals.

    # For all other reports, groups starts by grouping postings together by

    # invoice link string, then uses AccrualReport.make_consistent() to split

    # out groups that need it.