त्रपुजतुनोः षुक्

Operational rule vidhi

The affix अण् with the augment षुक् comes in the sense of 'its modification' after the words त्रपुन् and जतुन्.

Word by word (padaccheda)

Target त्रपु- जतुनोः ṣaṣṭhī · dvivacanaSubstitute / term षुक् prathamā · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
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

त्रपु-जतुनोः षुक्

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

As a rewrite rule (sketch from case endings)

त्रपु- जतुनोः → षुक्

Replace trapu-jatunoḥ with ṣuk. Inherited words (anuvṛtti) may add further conditions.

Commentary

The affix अण् with the augment षुक् comes in the sense of 'its modification' after the words त्रपुन् and जतुन्।
This debars अञ् 4.3.139. As, त्रपुणो विकारः = त्रापुषम्; जातुषम् 'modification of tin and lac.' In these words denoting non-organic beings, the sense of अवयव is not denoted by the affix.

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