श्नसोरल्लोपः

Operational rule vidhi

The अकार of the 'श्नम्' विकरण and of the verb root 'अस्' is removed when followed by a सार्वधातुक किति / ङित् प्रत्यय.

Word by word (padaccheda)

Target श्ना- असोः ṣaṣṭhī · dvivacanaSubstitute / term अल्लोपः 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

श्ना-असोः अल्लोपः

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)

श्ना- असोः → अल्लोपः

Replace śnā-asoḥ with allopaḥ. Inherited words (anuvṛtti) may add further conditions.

Commentary

Before a सार्वधातुक कित् or ङित् affix, the अ of श्न and अस् is elided.
श्न is the characteristic of the roots of the Rudhadi class, while अस् is the root itself. Thus रुन्धः; रुन्धति, भिन्तः, भिन्दन्ति; So also of अस् :- स्तः, सन्ति ॥ But भिनत्ति and अस्ति before non-कित् and non-ङित् affixes. श्न + अस् ought to be श्नास्, instead of that we have श्नस् in the sutra. The पररूप single substitution is an irregularity.

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