def parseEtreeTag(tag):
    """Return namespace, tagname pair.

      >>> parseEtreeTag('{DAV:}prop')
      ['DAV:', 'prop']

      >>> parseEtreeTag('{}prop')
      ['', 'prop']

      >>> parseEtreeTag('prop')
      (None, 'prop')

    """
    if tag[0] == "{":
        return tag[1:].split("}")
    return None, tag