ईदासः

Operational rule vidhi

ई is substituted for the आ of आन after आस्.

Word by word (padaccheda)

Substitute / term ईत् prathamā · ekavacanaLeft context आसः pañcamī · ekavacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.

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Full reading

ईत् आसः

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

Commentary

ई is substituted for the आ of आन after आस्।
Thus आसीनो यजते ॥ (See 1.1.54) by which the first letter of the second term after आसः which is exhibited in the Ablative is taken here). The आने which is in the 7th case in the last aphorism, should be taken in the 6th case in this.

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