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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_all_importing_namespace_hashes( self ):
 """ Get the set of all preordered and revealed namespace hashes that have not expired. """

        cur = self.db.cursor()
        namespace_hashes = namedb_get_all_importing_namespace_hashes( cur, self.lastblock )
        return namespace_hashes