Praudhamanorama p211

p211

Under 'dveratve satyāpi' [even when there is the a-substitution of dvi]: The idea is that the substitution of a for tyad and so forth, even though caused by conjunction (sannipāta) with a case ending, is a cause for ṭāp, because the metarule of conjunction (sannipātaparibhāṣā) is non-obligatory, and because 'na yāsayoḥ' (Pā. Sū. 7.3.45) is an indicative sign.
Under 'iyādeśaḥ' [the substitute iy]: The meaning is that the substitute iy that has been prescribed before affixes beginning with vowels—when the feminine word strī is present in the masculine as a secondary member (upasarjana)—occurs only before the four endings beginning with os, and not elsewhere, because it is blocked by the previously mentioned [operations] such as the absence of guṇa and of the augment nām.
Under 'neyaṅuvaṅsthānāvastrī' (Pā. Sū. 1.4.4): 'Those two in whose place they stand' are sthānau; the suffix lyuṭ is in the locative sense (adhikaraṇa). 'iyaṅuvaṅoḥ sthānau' is a genitive compound. He states the resultant meaning: 'Those two in whose place iyaṅ and uvaṅ abide.' This prohibition applies only where iyaṅ and uvaṅ are realized. Where, however, they would be blocked by an exception, there is no prohibition—for this purpose the word sthāna has been mentioned.
Under 'vāmi' (Pā. Sū. 1.4.5): Although the older grammarians say that the relevant prohibition is made optional by this rule, nonetheless, when a prohibition is made optional, what results is simply that the positive rule is made optional. And thus, for the sake of brevity (lāghava), that alone is appropriate as the meaning of the rule; with this in mind, he says: 'are optionally given the technical name nadī.' In the rule 'ṅiti hrasvaśca' (Pā. Sū. 1.4.6) as well, it is in this very way.