code: def _categorize(self, slug, category):
          
          key = self._category_key(category)
          self.r.sadd(key, slug)
  
          # Store all category names in a Redis set, for easy retrieval
          self.r.sadd(self._categories_key, category)
docs: Add the ``slug`` to the ``category``. We store category data as
          as set, with a key of the form::
  
              c:<category name>
  
          The data is set of metric slugs::
  
              "slug-a", "slug-b", ...