generate the PAULA file that contains the primary text of the document
        graph. (PAULA documents can have more than one primary text, but
        discoursegraphs only works with documents that are based on exactly one
        primary text.)

        Example
        -------
        <?xml version="1.0" standalone="no"?>
        <!DOCTYPE paula SYSTEM "paula_text.dtd">
        <paula version="1.1">
            <header paula_id="maz-1423.text" type="text"/>
            <body>Zum Angewöhnen ...</body>
        </paula>

def __gen_primary_text_file(self):
        """
        generate the PAULA file that contains the primary text of the document
        graph. (PAULA documents can have more than one primary text, but
        discoursegraphs only works with documents that are based on exactly one
        primary text.)

        Example
        -------
        <?xml version="1.0" standalone="no"?>
        <!DOCTYPE paula SYSTEM "paula_text.dtd">
        <paula version="1.1">
            <header paula_id="maz-1423.text" type="text"/>
            <body>Zum Angewöhnen ...</body>
        </paula>
        """
        paula_id = '{0}.{1}.text'.format(self.corpus_name, self.name)
        E, tree = gen_paula_etree(paula_id)
        tree.append(E.body(get_text(self.dg)))
        self.files[paula_id] = tree
        self.file2dtd[paula_id] = PaulaDTDs.text
        return paula_id