code: def normalize_subpath(self, subpath):
          
          if subpath is None:
              return None
  
          # Normalize the subpath
          subpath = posixpath.normpath(subpath)
  
          # Add or remove trailing slash to properly support relative links
          filename = os.path.normpath(safe_join(self.root_directory, subpath))
          if os.path.isdir(filename):
              subpath += '/'
  
          return subpath
docs: Normalizes the specified subpath, or None if subpath is None.
  
          This allows Readme files to be inferred from directories while
          still allowing relative paths to work properly.
  
          Raises werkzeug.exceptions.NotFound if the resulting path
          would fall out of the root directory.