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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 authkeywords(self):
 """List of author-provided keywords of the abstract."""

        keywords = self._json['authkeywords']
        if keywords is None:
            return None
        else:
            try:
                return [d['$'] for d in keywords['author-keyword']]
            except TypeError:  # Singleton keyword
                return [keywords['author-keyword']['$']]