仁王護國般若波羅蜜多經疏 2.30

2.30

解曰：言空空者，謂一切法空，此空由空空。言大空者，謂即十方，東南西北四維上下。此中東方由東方空，南等亦爾。勝義空者，勝義謂涅槃，此勝義由勝義空，二俗一真相性空故。從此第三為無為空。

The commentary states: As for "emptiness of emptiness," it means that all phenomena are empty, and this emptiness is empty through emptiness. As for "great emptiness," it refers to the ten directions: east, south, west, north, the four intermediate directions, above, and below. Here, the east is empty through the east, and the south and others are also like this. As for "ultimate emptiness," "ultimate" refers to nirvāṇa; this ultimate is empty through the ultimate, because two are conventional and one is real, and their characteristics and natures are empty. Next is the third [part], the emptiness of the conditioned and unconditioned.