सृपितृदोः कसुन्

Operational rule vidhi

In the छन्दस् (Vedas), the affix कसुन् comes after the verbs 1. सृप् 'to creep' 2. तृद् 'to injure' , in the sense of Infinitives indicating name of action.

Word by word (padaccheda)

Target सृपि- तृदोः ṣaṣṭhī · dvivacanaSubstitute / term कसुन् 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.

Dotted words are technical terms. Hover or tap one to see its card.

Full reading

सृपि-तृदोः कसुन्

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)

सृपि- तृदोः → कसुन्

Replace sṛpi-tṛdoḥ with kasun. Inherited words (anuvṛtti) may add further conditions.

Commentary

In the छन्दस् (Vedas), the affix कसुन् comes after the verbs 1. सृप् 'to creep' 2. तृद् 'to injure' , in the sense of Infinitives indicating name of action.
Thus विसृपः । पुरा क्रूरस्य विसृपः (Yaj. I.28.); आतृदः (Rig. VIII.I.12); पुरा अत्रुभ्य आतृदः (Rig. VIII.I.12). These words are Indeclinable by 1.1.40.

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