इञः प्राचाम्

Operational rule vidhi

After a word ending in the गोत्र affix -- (4.1.95), there is elision of लुक् of the युवन् affix, when the गोत्र of the people called प्राच्य 'Eastern' is denoted.

Word by word (padaccheda)

Left context इञः pañcamī · ekavacanaTarget प्राचाम् ṣaṣṭhī · bahuvacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

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Full reading

इञः प्राचाम्

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

Commentary

After a word ending in the गोत्र affix -- 4.1.95, there is elision of लुक् of the युवन् affix, when the गोत्र of the people called प्राच्य 'Eastern' is denoted.
पान्नागर + इञ् = पान्नागारिः 'the Gotra descendent of Pannagara'; after this the Yuvan affix फक 4.1.101 is elided by the present rule, and so पान्नागारिः is both the father and the son. So मान्यरैषणिः father and son. The word प्राचाम् here qualifies the word Gotra understood and does not show that it is an optional rule in the opinion of Eastern Grammarians. When the Gotra of Prachyas is not denoted, there is no elision. Thus दाक्षिः and दाक्षायणः 'son of Dakshi.'

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