docs: def dirname(cls, namespace):
          """convert a namespace url to a directory name. 
              Also accepts an Element 'tag' with namespace prepended in {braces}."""
          md = re.match("^\{?(?:[^:]+:/{0,2})?([^\}]+)\}?", namespace)
          if md is not None:
              dirname = re.sub("[/:]", '_', md.group(1))
          else:
              dirname = ''
          return dirname