कास्प्रत्ययादाममन्त्रे लिटि

Operational rule vidhi

आम् is the affix of the verb कास् 'to cough' and of those roots that are formed by affixes i.e. the derivative verbs, when लिट् (Perfect Tense) follows, in the मन्त्र.

Word by word (padaccheda)

Left context कास्प्रत्ययात् pañcamī · ekavacanaSubstitute / term आम् prathamā · ekavacanaRight context अमन्त्रे saptamī · ekavacanaRight context लिटि saptamī · ekavacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").

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Full reading

कास्प्रत्ययात् आम् अमन्त्रे लिटि

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

Commentary

आम् is the affix of the verb कास् 'to cough' and of those roots that are formed by affixes i.e. the derivative verbs, when लिट् (Perfect Tense) follows, in the मन्त्र।
This forms the Periphrastic Perfect. The Perfect of कास् will be कासाम्, after which the terminations of the Perfect are all elided 2.4.81 and instead, the perfect tense of the auxiliary verb भू or अस् is added. As कासाञ्चक्रे 'he coughed'; चोरयामास 'he stole'. So also लोलूवाञ्चक्रे.

This rule however does not apply to the Vedas; as कृष्णो नोनाव Rig. Veda (I.79.2).

Vart:- This rule also applies where the root is polysyllabic consisting of more than one vowel; as चुलुम्पाञ्चकार, दरिद्राञ्चकार ॥

The final म् of आम् is not indicatory.

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