code: def sprite_file(map, sprite):
      
      map = map.render()
      sprite_maps = _get_cache('sprite_maps')
      sprite_map = sprite_maps.get(map)
      sprite_name = String.unquoted(sprite).value
      sprite = sprite_map and sprite_map.get(sprite_name)
      if not sprite_map:
          log.error("No sprite map found: %s", map, extra={'stack': True})
      elif not sprite:
          log.error("No sprite found: %s in %s", sprite_name, sprite_map['*n*'], extra={'stack': True})
      if sprite:
          return String(sprite[1][0])
      return String.unquoted('')
docs: Returns the relative path (from the images directory) to the original file
      used when construction the sprite. This is suitable for passing to the
      image_width and image_height helpers.