निरभ्योः पूल्वोः

Operational rule vidhi

The affix घञ् comes after the roots 1. पू 'to purify' 2. लू 'to cut' when निर् and अभि are respectively in composition.

Word by word (padaccheda)

Right context निर्- अभ्योः saptamī · dvivacanaTarget पू- ल्वोः ṣaṣṭhī · 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").
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

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Full reading

निर्-अभ्योः पू-ल्वोः

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

The affix घञ् comes after the roots 1. पू 'to purify' 2. लू 'to cut' when निर् and अभि are respectively in composition.
The word पू refers to both पूङ् and पूञ्. The rule 1.3.10 must be applied here, and thus निर् comes before पू, and अभि before लू । As निष्पावः 'winnowing'; अभिलावः 'moving'. But पवः and लवः, without these prepositions.

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