The offset of *category* in the overall sequence of leaf categories.
        A non-leaf category gets the index of its first sub-category.

def index(self, category):
        """
        The offset of *category* in the overall sequence of leaf categories.
        A non-leaf category gets the index of its first sub-category.
        """
        index = 0
        for this_category in self._categories:
            if category is this_category:
                return index
            index += this_category.leaf_count
        raise ValueError('category not in top-level categories')