Generate a ``tuple`` object for each leaf category in this
        collection, containing the leaf category followed by its "parent"
        categories, e.g. ``('San Francisco', 'CA', 'USA'). Each tuple will be
        the same length as the number of levels (excepting certain edge
        cases which I believe always indicate a chart construction error).

def _iter_flattened_categories(self):
        """
        Generate a ``tuple`` object for each leaf category in this
        collection, containing the leaf category followed by its "parent"
        categories, e.g. ``('San Francisco', 'CA', 'USA'). Each tuple will be
        the same length as the number of levels (excepting certain edge
        cases which I believe always indicate a chart construction error).
        """
        levels = self.levels
        if not levels:
            return
        leaf_level, remaining_levels = levels[0], levels[1:]
        for category in leaf_level:
            yield self._parentage((category,), remaining_levels)