code: def _crawl_oxford_and_find_main_xml(self):
          
          self.found_articles = []
  
          def visit(arg, dirname, names):
              files = [filename for filename in names if ".xml" in filename]
              if files:
                  try:
                      for f in files:
                          self.found_articles.append(join(dirname, f))
  
                  except Exception as err:
                      register_exception()
                      print("ERROR: can't normalize %s: %s" % (dirname, err),
                            file=sys.stderr)
  
          if hasattr(self, 'path_unpacked'):
              walk(self.path_unpacked, visit, None)
          elif self.path:
              walk(self.path, visit, None)
          else:
              self.logger.info("Nothing to do.")
docs: A package contains several subdirectory corresponding to each article.
          An article is actually identified by the existence of a main.pdf and
          a main.xml in a given directory.