docs: def _get_aliases(self):
          u"""Creates a dict with aliases.
  
          The key is a normalized tagname, value the original tagname.
          """
          if self._aliases is None:
              self._aliases = {}
  
              if self._xml is not None:
                  for child in self._xml.iterchildren():
                      self._aliases[helpers.normalize_tag(child.tag)] = child.tag
  
          return self._aliases