code: def index(self, sub_category):
          
          index = self._parent.index(self)
          for this_sub_category in self._sub_categories:
              if sub_category is this_sub_category:
                  return index
              index += this_sub_category.leaf_count
          raise ValueError('sub_category not in this category')
docs: The offset of *sub_category* in the overall sequence of leaf
          categories.