Because the regular keys iteration of FIRSTKEY and NEXTKEY is only over the primary keys, the secondary_keys function is provided for the secondary mapping.

由于FIRSTKEY和NEXTKEY的常规keys迭代只是对主键进行，因此secondary_keys函数是为第二个映射提供的。

youdao

Here we present a new method for predicting RNA secondary structures including pseudoknots through iteration.

文章提出一种基于迭代法预测带伪结rna二级结构的新方法。

youdao

The secondary, front-facing camera, meanwhile, has a 2-megapixel iteration of Sony's Exmor R sensor.

ENGADGET: Sony Xperia Z: hands-on with the new flagship at CES 2013 (video)