code: def __gen_primary_text_file(self):
          
          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
docs: 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>