शकि णमुल्कमुलौ

Operational rule vidhi

The affixes णमुल् and कमुल् are added to roots in the छन्दस् (Vedas) to form Infinitives, when they are governed by the verb शक् 'to be able'.

Word by word (padaccheda)

Right context शकि saptamī · ekavacanaSubstitute / term - औ prathamā · dvivacana

Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

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Full reading

शकि णमुल्-कमुलौ

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Commentary

The affixes णमुल् and कमुल् are added to roots in the छन्दस् (Vedas) to form Infinitives, when they are governed by the verb शक् 'to be able'.
Of the affix णमुल् the real affix is अम्; the letter ण् causes vriddhi 7.2.115; and ल regulates the accent 6.1.193. So also of कमुल्, the letter क् prevents guna and vriddhi substitution 1.1.5.

Thus अग्निं वै देवा विभाजं नाशक्नुवन् 'the Gods were not able to divide Agni'. विभज् +णमुल् = विभाजं = विभक्तुम् । So also अपलुपं नाशक्नुवन्, instead of अपलोप्तुम् ।

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.