The penultimate vowels of a aṅga (stem) ending in in , han , pūṣan or aryaman are lengthened before the su of the Nominative 1st-Case singular but not in Vocative Case singular.,
Thus दण्डी, वृत्रहा, पूषा, अर्यमा ॥ The न् is elided by 8.2.7, and the case-affix by 6.1.68. in the Vocative singular we have हे दण्डिन्, हे पूषन्, हे वृत्रहन्, हे अर्यमन् ॥,