Given a list of dividends whose ex_dates are all the next trading
        day, calculate and store the cash and/or stock payments to be paid on
        each dividend's pay date.

        Parameters
        ----------
        cash_dividends : iterable of (asset, amount, pay_date) namedtuples

        stock_dividends: iterable of (asset, payment_asset, ratio, pay_date)
            namedtuples.

def earn_dividends(self, cash_dividends, stock_dividends):
        """Given a list of dividends whose ex_dates are all the next trading
        day, calculate and store the cash and/or stock payments to be paid on
        each dividend's pay date.

        Parameters
        ----------
        cash_dividends : iterable of (asset, amount, pay_date) namedtuples

        stock_dividends: iterable of (asset, payment_asset, ratio, pay_date)
            namedtuples.
        """
        for cash_dividend in cash_dividends:
            self._dirty_stats = True  # only mark dirty if we pay a dividend

            # Store the earned dividends so that they can be paid on the
            # dividends' pay_dates.
            div_owed = self.positions[cash_dividend.asset].earn_dividend(
                cash_dividend,
            )
            try:
                self._unpaid_dividends[cash_dividend.pay_date].append(div_owed)
            except KeyError:
                self._unpaid_dividends[cash_dividend.pay_date] = [div_owed]

        for stock_dividend in stock_dividends:
            self._dirty_stats = True  # only mark dirty if we pay a dividend

            div_owed = self.positions[
                stock_dividend.asset
            ].earn_stock_dividend(stock_dividend)
            try:
                self._unpaid_stock_dividends[stock_dividend.pay_date].append(
                    div_owed,
                )
            except KeyError:
                self._unpaid_stock_dividends[stock_dividend.pay_date] = [
                    div_owed,
                ]