प्रशस्यस्य श्रः

Operational rule vidhi

For प्रशस्य is substituted श्र when the affixes इष्ठन् and इयसुन् follow.

Word by word (padaccheda)

Target प्रशस्यस्य ṣaṣṭhī · ekavacanaSubstitute / term श्रः 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, dashed ones pratyāhāras (names for classes of sounds or affixes). Hover or tap one to see its card.

Full reading

प्रशस्यस्य श्रः

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)

प्रशस्यस्य → श्रः

Replace praśasyasya with śraḥ. Inherited words (anuvṛtti) may add further conditions.

Commentary

For प्रशस्य is substituted श्र when the affixes इष्ठन् and इयसुन् follow.
Though प्रशस्य is not an attribute, it takes these affixes by the application of this rule. The anuvritti of अजादी runs here, but the word must be changed into locative for the purposes of this aphorism, and which will read thus प्रशस्यस्य श्रोऽजाद्योः ॥ Thus सर्व इमे प्रशस्या अयमेषामतिशयेन प्रशस्यः = श्रेष्ठः, उभाविमौ प्रशस्यौ, अयमनयोरतिशयेन प्रशस्यः = श्रेयान् ॥ Rule 6.4.163 prevents the elision of the अ of श्र, which otherwise would have been elided by 6.4.148 and 5.4.155. These words are equal to प्रशस्यतरः and प्रशस्यतमः ॥

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