āt (ā) is substituted for the final of 1. pathin 2. mathin and 3. ṛbhukṣin before the ending su (Nominative 1st-Case and Vocative Case singular).,
As पन्थाः, मन्थाः, ऋभुक्षाः ॥ Though the sthanin here is a nasal (i.e. म्), yet the substitute आ is not to be nasalised, but to be pronounced purely. For न्थ see (VII. I. 87). The nasal आ is not to be taken on the maxim भाव्यमानेन सवर्णानां ग्रहणं न भवति ॥ \A letter which is taught in a rule does not denote the letters homogeneous with it.\,