docs: def idx(self):
          """
          Return partname index as integer for tuple partname or None for
          singleton partname, e.g. ``21`` for ``'/ppt/slides/slide21.xml'`` and
          |None| for ``'/ppt/presentation.xml'``.
          """
          filename = self.filename
          if not filename:
              return None
          name_part = posixpath.splitext(filename)[0]  # filename w/ext removed
          match = self._filename_re.match(name_part)
          if match is None:
              return None
          if match.group(2):
              return int(match.group(2))
          return None