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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 _issuer(self, entityid=None):
 """ Return an Issuer instance """

        if entityid:
            if isinstance(entityid, Issuer):
                return entityid
            else:
                return Issuer(text=entityid, format=NAMEID_FORMAT_ENTITY)
        else:
            return Issuer(text=self.config.entityid,
                          format=NAMEID_FORMAT_ENTITY)