code: def _find(self, root, tagname, id=None):
          
          if id is None:
              result = root.find('.//%s' % tagname)
              if result is None:
                  raise LookupError('Cannot find any %s elements' % tagname)
              else:
                  return result
          else:
              result = [
                  elem for elem in root.findall('.//%s' % tagname)
                  if elem.attrib.get('id', '') == id
              ]
              if len(result) == 0:
                  raise LookupError('Cannot find a %s element with id %s' % (tagname, id))
              elif len(result) > 1:
                  raise LookupError('Found multiple %s elements with id %s' % (tagname, id))
              else:
                  return result[0]
docs: Returns the first element with the specified tagname and id