code: def get_subtree(self, root):
          
          if not isinstance(root, Tree):
              self.log_exc(u"root is not an instance of Tree", None, True, TypeError)
          new_text_file = TextFile()
          new_text_file.fragments_tree = root
          return new_text_file
docs: Return a new :class:`~aeneas.textfile.TextFile` object,
          rooted at the given node ``root``.
  
          :param root: the root node
          :type  root: :class:`~aeneas.tree.Tree`
          :rtype: :class:`~aeneas.textfile.TextFile`