सन्यतः

Operational rule vidhi

इ is substituted for the final short अ of the reduplicate in the Desiderative.

Word by word (padaccheda)

Right context सनि saptamī · dvivacanaTarget अतः ṣaṣṭhī · ekavacana

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

सनि अतः

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

Commentary

इ is substituted for the final short अ of the reduplicate in the Desiderative.
As पिपक्षति, यियक्षति, 8.2.36, 8.2.41 तिष्ठासति, पिपासति ॥ Why do we say in the Desiderative? Observe पपाच ॥ Why do we say "for the अ"? Observe लुलूषति ॥ Why do we say 'short अ'? Observe पापचिषते the Desiderative of the Intensive पापच्यते ॥

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