6.1.113
अतो रोरप्लुतादप्लुते
Operational rule vidhi
A रुँ letter that is sandwiched between an अप्लुत-ह्रस्व-अकार from each side is converted to an उकार.
Word by word (padaccheda)
Left context अतः pañcamī · ekavacanaTarget रोः ṣaṣṭhī · ekavacanaLeft context अप्लुतात् pañcamī · ekavacanaRight context अप्लुते 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.
- Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
- 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").
Full reading
अतः रोः अप्लुतात् अप्लुते
Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.
Commentary
The उ is the substitute of रु (the र् substitute of a final स् 8.2.66) when an अ , which is not a प्लुत, both precedes and follows it.
The phrase उत् is understood here, as well as the word आत् of 6.1.109, 6.1.111. Thus वृक्ष + सु (Ist singular) = वृक्षस् = वृक्षर् 8.2.66. वृक्षर् + अत्र = वृक्षउ + अत्र = वृक्षो + अत्र 6.1.87 = वृक्षोऽत्र 6.1.109; so also प्लक्षोऽत्र ॥ This ordains उ for र्, whereby 8.3.17, there would have been otherwise य; and this उ does not become asiddha (as it depends upon रु 8.2.66) for the purposes of 8.3.17, as it otherwise would have been by 8.2.1.
Why do we say 'after an अ'? Observe अग्निर् + अत्र = अग्निरत्र ॥ Why do say 'a short अ'? Observe वृक्षाः + अत्र = वृक्षा अत्र ॥ Why do we use रु with its indicatory उ, and not use the र् generally? Observe स्वर् + अत्र = स्वरत्र, प्रातर् + अत्र = प्रातरत्र ॥ Here the final र् is part of the words, and is not produced from स् ॥ The word अति is understood here also from 6.1.109. The र must be followed by a short अ, therefore, not here: वृक्षर् + इह = वृक्ष इह ॥ The subsequent अ must be short, the rule does not apply here वृक्षर् + आश्रितः = वृक्ष आश्रितः ॥ Why do we say 'preceded by an apluta अ' ? The rule will not apply if a Pluta vowel precedes it. As, सुस्रोता ३ अत्र त्वमसि 8.2.84. Why do we say "when followed by an apluta अ"? Observe तिष्ठतु पय आ३ग्निदत्त 8.2.86. Here प्लुत being held asiddha, there would have been उ substitution, had not the phrase अप्लुते been used in the aphorism.
Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.