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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 get_identifier(self):
 """Validate and returns disqus_identifier option value. :returns: disqus_identifier config value. :rtype: str """

        if 'disqus_identifier' in self.options:
            return self.options['disqus_identifier']

        title_nodes = self.state.document.traverse(nodes.title)
        if not title_nodes:
            raise DisqusError('No title nodes found in document, cannot derive disqus_identifier config value.')
        return title_nodes[0].astext()