Before the case-ending os and before ā of the Instrumental -3rd case, e is substituted for the final ā of the feminine affix.,
आङ् is the name given to the affix टा, the Instrumental Singular by the ancient grammarians. As खट्वया, मालया, खट्वयोः, मालयोः, बहुराजया, कारीषगन्ध्यया, बहुराजयोः, कारीषगन्ध्ययोः ॥ Why the आ of the Feminine affix only? Observe कीलालपा ब्राह्मणेन कीलालपो ब्राह्मणकुलयोः ॥ Where ever ङी or आप् is employed in Grammar, they mean the long forms ई and आ, and not when they are shortened, therefore, not here, अतिखट्वेन ब्राह्मणकुलेन ॥
The word कीलालपा is derived from कीलालं पिबति with the affix विच् 3.2.74. In the Instrumental Singular the final आ is elided by 6.4.140. Had therefore, आ only been used in the sutra, instead of आप्, there would have been ए substitution in the case of कीलालपा also; for the लोप् rule 6.4.140 would find its scope in कीलालपः पश्य &c, and would be debarred here by the present sutra.
The maxim ङ्याब् ग्रहणेऽदीर्घग्रहणम् is necessary, because otherwise on the maxim of sthanivadbhava, the short substitutes of ङी and आप् would also be included. In fact, in the sutra prohibiting sthanivad bhava, we find this vartika ङ्याब् ग्रहणेऽदीर्घः \A short (not long) substitute of ङा and आप् is not sthanivat\.,