अचि विभाषा

र् is optionally changed to ल् before an affix beginning with a vowel.
As निगिरति or निगिलति, निगरणम् or निगलनम्, निगारकः or निगालकः ॥

This is a vyavasthita-vibhasha, the optional forms have particular meanings. Thus गलः meaning 'neck' is always with ल; while गरः 'poison' is always with र.

In निगार्यते or निगाल्यते, the elision of णि is considered sthanivad, and hence this option, though the actual affix begins with य ॥ Objection:- The sthanivad-bhava rule is invalid here by 8.2.1. Auswer: The rule पूर्वत्रासिद्धं does not hold good with regard to the rules of संयोगादिलोप, लत्व and णत्व on the maxim "तस्य दोषः संयोगादिलोपलत्वणत्वेषु" ॥

Or the र् will be first changed to ल्, as being antaranga, and then the णि will be elided.

The forms गिरौ, गिरः are either from the Kryadi root gri, or l-change has not taken place on the maxim धातोः स्वरूपग्रहणे तत्प्रत्यये विज्ञानम् and as the affixes औ and अः are not affixes which are ordained after a verb, but are affixes added to nouns, hence the ल change has not taken place. In fact the words 'an affix beginning with a vowel' in the sutra, means "a verbal affix beginning with a vowel," and not a noun affix. These are the Dual and Plural of the Nominative case of गॄ formed with the affix क्विप् ॥