अध्वनो यत्खौ

Operational rule vidhi

The affixes यत् (यं) and ख (ईन) come in the sense of 'who is fit or able to follow' after the word अध्वन्.

Word by word (padaccheda)

Left context अध्वनः pañcamī · ekavacanaSubstitute / term य - खौ prathamā · dvivacana

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.

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Full reading

अध्वनः यत्-खौ

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

Commentary

The affixes यत् (यं) and ख (ईन) come in the sense of 'who is fit or able to follow' after the word अध्वन्।
Thus अध्वानमलंगामी = अध्वनीनः or अध्वन्यः 'able to undertake a journey'. By aphorisms 6.4.168, 6.4.169, the word अध्वन् retains its original form before the affix य and ख, not losing its final अन् which it does before other affixes, by 6.4.144.

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