au is substituted for ṇal , the affix of the 1st and 3rd person singular of the liṭ (Perfect Tense), after the roots ending in long ā,
Thus पपौ, तस्थौ, जग्लौ, मम्लौ, from पा, स्था, ग्लै (ग्ला) and म्लै (म्ला) ॥ The form पपौ is thus evolved, पा + णल् = पा + अ ॥ Here three operations simultaneously present themselves for application, namely; 1. Reduplication, 2. Lengthening; by the single substitution of one long vowel आ, for आ + अ; and 3. The substitution of औ for आ by the present sutra. In what order should these operations be then performed? First the affix अ should be replaced by औ, then the single substitution of औ for आ + औ ; and then treating it as sthanivat, and then reduplication. For if the single substitution of आ for आ + अ had taken place first, then there would be no scope for औ substitution, therefore the औ substitution, should take place first. Thus we have पा + औ = पौ; this vriddhi-ekadesa, is treated as sthanivat by 1.1.59, and it causes reduplication. The vriddhi-ekadesa should first take place (i. e. आ + औ = औ) and then the reduplication, because the former rule is subsequent to the latter.,