The affix ṇa is optionally used after the verb graha 'to seize'.,
This debars the affix अच्. Thus ग्रह् + ण = ग्रह् + अ 7.2.116 = ग्राहः 'seizing', 'a crocodile'; or ग्रह + अच् =ग्रहः 'a planet'.
The option allowed by this aphorism is a vyavasthita-vibhasha or definite option, that is to say, the affix ण is always used when the word to be formed means a water-animal, such as a crocodile, shark &c., and in that sense the word is ग्राह. But not so, when heavenly luminaries are meant; there the word is ग्रह.
Vart:- This rule should be extended to the root भू 'to be' also. Thus:- भावः 'condition' and भवः 'existence'.,