docs: def _sprite_url(map):
      """
      Returns a url to the sprite image.
      """
      map = StringValue(map).value
      sprite_map = sprite_maps.get(map)
      if not sprite_map:
          log.error("No sprite map found: %s", map)
      if sprite_map:
          url = '%s%s?_=%s' % (ASSETS_URL, sprite_map['*f*'], sprite_map['*t*'])
          url = "url(%s)" % escape(url)
          return StringValue(url)
      return StringValue(None)