वनगिर्योः संज्ञायां कोटरकिंशुलकादीनाम्

Operational rule vidhi

For the final vowel of कोटर etc. a long vowel is substituted before वन and also of किंशुलक etc. before गिरि when the compound is a name.

Word by word (padaccheda)

Right context वन- गिर्योः saptamī · dvivacanaRight context संज्ञायाम् saptamī · ekavacanaTarget कोटर- किंशुलकादीनाम् ṣaṣṭhī · bahuvacana

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

वन-गिर्योः संज्ञायाम् कोटर-किंशुलकादीनाम्

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

Commentary

For the final vowel of कोटर etc. a long vowel is substituted before वन and also of किंशुलक etc. before गिरि when the compound is a name.
Thus कोटरावणम्, मिश्रकावणम्, सिध्रकावणम्, सारिकावणम् ॥

So also किंशुलकागिरिः, अञ्जनागिरिः, &c.

The न is changed to ण in वन by 8.4.4. But असिपत्रवनम् and कृष्णगिरि as these words do not belong to the above classes.

1 कोटर, 2 मिश्रक, 3 सिध्रक, 4 पुरग (पुरक), 5 शारिक (सारिक)॥

1 किंशुलुक (किंशुलक), 2 शाल्व (साल्वक), 3 नड, 4 अञ्जन, 5 मञ्जन, 6 लोहित, 7 कुक्कुट ॥

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