code: def index_filename_rel_other_index(self, other: str) -> str:
          
          return relpath(self.index_filename, start=dirname(other))
docs: Returns the filename of this index, relative to the director of another
          index. (For inserting a reference to this index into ``other``.)
  
          Args:
              other: the other index
  
          Returns:
              relative filename of our index