docs: def get_mirror(self, target_root, source_root=None):
          """
          Returns a File or Folder object that reperesents if the entire
          fragment of this directory starting with `source_root` were copied
          to `target_root`.
  
          >>> Folder('/usr/local/hyde/stuff').get_mirror('/usr/tmp',
                                                  source_root='/usr/local/hyde')
          Folder('/usr/tmp/stuff')
          """
          fragment = self.get_relative_path(
                          source_root if source_root else self.parent)
          return Folder(target_root).child(fragment)