पथिमथ्यृभुक्षामात्

Operational rule vidhi

The words पथिन्, मथिन् and ऋभुक्षिन् get an 'आ' आदेश in presence of the सुँ-प्रत्यय.

Word by word (padaccheda)

Target पथिमथ्यृभुक्षाम् ṣaṣṭhī · bahuvacanaSubstitute / term आत् 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

पथिमथ्यृभुक्षाम् आत्

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)

पथिमथ्यृभुक्षाम् → आत्

Replace pathimathyṛbhukṣām with āt. Inherited words (anuvṛtti) may add further conditions.

Commentary

आत् (आ) is substituted for the final of 1. पथिन् 2. मथिन् and 3. ऋभुक्षिन् before the ending सु (Nominative 1st-Case and Vocative Case singular).
As पन्थाः, मन्थाः, ऋभुक्षाः ॥ Though the sthanin here is a nasal (i.e. म्), yet the substitute आ is not to be nasalised, but to be pronounced purely. For न्थ see 7.1.87. The nasal आ is not to be taken on the maxim भाव्यमानेन सवर्णानां ग्रहणं न भवति ॥ "A letter which is taught in a rule does not denote the letters homogeneous with it."

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