विभाषा धेट्श्व्योः

Operational rule vidhi

For the verbs धेट् and श्वि , the च्लि प्रत्यय is optionally converted to चङ्.

Word by word (padaccheda)

Substitute / term prathamā · ekavacanaTarget धेट्- श्व्योः ṣaṣṭhī · dvivacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

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Full reading

धेट्-श्व्योः

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)

धेट्- श्व्योः → optional

Replace dheṭ-śvyoḥ with vibhāṣā, optionally. Inherited words (anuvṛtti) may add further conditions.

Commentary

After the roots धे 'to suck' and श्वि 'to grow', चङ् is optionally the substitute of च्लि , when लुङ् (Aorist) follows signifying an agent.
As, अदधत्, 'he sucked'. When it takes the alternative सिच्, then rule 2.4.78 comes into operation, and we have अधात् and अधासीत् 'he sucked'. So also of श्वि, we have अशिश्वियत् 'he grew'. It also takes the अङ्, 3.1.58 and then its form is अश्वत् or अश्वयीत्.

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