def retag_string(self, string, tags):
        """substitutes each tag back into string, into occurrences of <0>, <1> etc"""
        for i, tag in enumerate(tags):
            bracketed = '<%s>' % i
            string = re.sub(bracketed, tag, string, 1)
        return string
substitutes each tag back into string, into occurrences of <0>, <1> etc