The affixes vini and ini come with the force of matup after the words tapas and sahara respectively.,
The word तपस् ends in अस्, and therefore by 5.2.121, it would have taken विनि; similarly सहस्र ends in अ, and by 5.2.115, it would have taken इनि; it may, therefore, be objected, where is the necessity for the present sutra? To this it is replied, that by the next sutra, these words take the affix अण् also; so that, had there not been this present sutra, the अण् would have debarred the application of इनि and विनि of 5.2.115 and 5.2.121. The affix ठन् 5.1.115 is also debarred after the word सहस्र, though it ends in अ ॥
Thus तपोऽस्यास्मिन् वा विद्यते = तपस्विन् nom. तपस्वी ॥ Similarly सहस्रिन् nom. सहस्रा ॥,