6.3.67
अरुर्द्विषदजन्तस्य मुम्
Operational rule vidhi
मुम् is the augment added immediately after the final vowel of अरुस , द्विषत् and of a stem ending in a vowel unless it is an indeclinable, when the word formed by a खित् affix follows.
Word by word (padaccheda)
Target अरुस्- द्विषस्- अजन्तस्य ṣaṣṭhī · ekavacanaSubstitute / term मुम् 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.
Full reading
अरुस्-
Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.
अरुस्-
Replace arus-dviṣas-ajantasya with mum. Inherited words (anuvṛtti) may add further conditions.
Commentary
मुम् is the augment added immediately after the final vowel of अरुस , द्विषत् and of a stem ending in a vowel unless it is an indeclinable, when the word formed by a खित् affix follows.
Thus अरुन्तुदः, द्विषन्तपः, कालिंमन्या &c. See 3.2.35, 3.2.39, 3.2.83. The स् of अरुस् and the त् of द्विषत् are dropped by 8.2.23.
Why do we say 'of अरुस् &c'? Observe विद्वन्मन्या ॥ Why do we say 'not an Indeclinable'? Observe दोषामन्यमहः, दिवामन्यारात्रिः ॥ Why do we use the word अन्त in अजन्त, when merely saying अचः would have sufficed by the rule of tadanta-vidhi? This indicates that the shortening taught in the preceding sutra does not debar the मुम् augment, nor does मुम् debar the shortening. So that मुम् is added after the shortening has taken place.
Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.