Add the given text fragment as the first or last child of the root node
        of the text file tree.

        :param fragment: the text fragment to be added
        :type  fragment: :class:`~aeneas.textfile.TextFragment`
        :param bool as_last: if ``True`` append fragment, otherwise prepend it

def add_fragment(self, fragment, as_last=True):
        """
        Add the given text fragment as the first or last child of the root node
        of the text file tree.

        :param fragment: the text fragment to be added
        :type  fragment: :class:`~aeneas.textfile.TextFragment`
        :param bool as_last: if ``True`` append fragment, otherwise prepend it
        """
        if not isinstance(fragment, TextFragment):
            self.log_exc(u"fragment is not an instance of TextFragment", None, True, TypeError)
        self.fragments_tree.add_child(Tree(value=fragment), as_last=as_last)