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        <p>See the function documentation for the correct usage of b_truncate parameter.</p>
        <p>The following four member functions have default parameter b_truncate, because they require all key/data pairs in the database be deleted before the real operation, and by default we use Db::truncate to truncate the database rather than delete the key/data pairs one by one, but Db::truncate requirs no open cursors on the database handle, and the four member functions will close any open cursors of 