Return fully qualified (Clark notation) tagname corresponding to
    short-form prefixed tagname *tag*.

def qtag(tag):
    """
    Return fully qualified (Clark notation) tagname corresponding to
    short-form prefixed tagname *tag*.
    """
    prefix, tagroot = tag.split(':')
    uri = nsmap[prefix]
    return '{%s}%s' % (uri, tagroot)