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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 oaiset(self):
 """Return the corresponding OAISet for given community. If OAIServer is not installed this property will return None. :returns: returns OAISet object corresponding to this community. :rtype: `invenio_oaiserver.models.OAISet` or None """

        if current_app.config['COMMUNITIES_OAI_ENABLED']:
            from invenio_oaiserver.models import OAISet
            return OAISet.query.filter_by(spec=self.oaiset_spec).one()
        else:
            return None