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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def reference_year(self, index):
 """Return the reference publication year."""

        # TODO Use meta:parsedDate field instead?
        ref_date = self.reference_date(index)
        try:
            # NB: datetime.year returns an int.
            return parse(ref_date).year
        except ValueError:
            matched = re.search(r"\d{4}", ref_date)
            if matched:
                return int(matched.group())
            else:
                return ""