Praudhamanorama p161

p161

[On the words] "does not exist" (*nāstīti*): the idea is, due to lack of usage (*anabhidhāna*). And thus, because the dual does not arise after the word *ubhaya*, when it would follow that it is an indeclinable (*avyaya*) on the ground of not having all case endings, the Mahābhāṣya passage under the rule *taddhitaścāsarvavibhaktiḥ* (Pā. 1.1.38)—"and the mention of *kṛt* and *taddhita* affixes is within the enumeration"—has been introduced by Kaiyaṭa.
[On the word] "exists" (*astīti*): the idea is that since the mention [in that list] is exhausted by barring forms like *pacatikalpam*, *pacatirūpam*, etc., there is no authority for [assuming] non-usage.
[On the words] "of that which ends in the affix *tayap*" (*tayappratyayāntasyeti*): the idea is, of the word *ubhaya*.
[On the words] "only obligatory" (*nityaiveti*): but [the usage] *pare'vara ubhayā amitrāḥ* is to be understood as Vedic.