राल्लोपः

Operational rule vidhi

After र् there is the elision of छ् and व् before क्वि and झलादि ङित् and कित् affixes -- अनुनासिकस्य क्विझलोः क्ङिति (6.4.15).

Word by word (padaccheda)

Left context रात् pañcamī · ekavacanaSubstitute / term prathamā · ekavacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
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

रात्

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

Commentary

After र् there is the elision of छ् and व् before क्वि and झलादि ङित् and कित् affixes -- अनुनासिकस्य क्विझलोः क्ङिति 6.4.15.
Thus from मुर्छा — मूर्, मुरौ, मुरः, मूर्तः, मूर्तवान्, मूर्तिः, the त of Nishtha is not changed into न by 8.2.57. So also from हुर्छा we have हूर्, हुरौ, हुरः, हूर्णः, हूर्णवान्, हूर्त्तिः ॥ So also the व is elided, as from तूर्वीः - तूर्, तुरौ, तुरः, तूर्णः, तूर्णवान्, तूर्त्तिः ॥ धुर्वी :- धुः, धुरौ, धुरः, धूर्णः, धूर्णवान्, धूर्त्ति ॥

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