8.3.102
निसस्तपतावनासेवने
Operational rule vidhi
ष् is substituted for the स् of निस् before the verb तप् when meaning is not that of 'repeatedly making red hot'.
Word by word (padaccheda)
Target निसः ṣaṣṭhī · ekavacanaRight context तपतौ saptamī · ekavacanaRight context अनासेवने saptamī · ekavacana
- 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
निसः तपतौ अनासेवने
Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.
Commentary
ष् is substituted for the स् of निस् before the verb तप् when meaning is not that of 'repeatedly making red hot'.
The word आसेवन means doing a thing repeatedly and here it means making it red-hot repeatedly. Thus निष्टपतिः सुवर्णम् = सकृत् अग्निम् स्पर्शयति i.e. he puts the gold into fire only once.
Why do we say when not meaning repeatedly. Observe निस्तपति र्सुवर्ण सुवर्णकारः = पुनः पुनरग्निं स्पर्शयति ॥
In the sentences निष्टप्तं रक्षः, निष्टप्ता अरातयः the change has taken place either as a Vedic irregularity or because the sense of repeatedness is not implied here.
Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.