v and y (preceded by a or ā and the end of a pada) and followed by u , when it is a word, are elided necessarily.,
The particle उ is a full pada or word. That Particle is meant here by the word उञ्, and not the उञ्, which is a root obtained by the samprasarana of वेञ् ॥ Thus स उ एकविंशतिः, स उ एकाग्निः ॥
Why do we use the word पदे \उ when it is a pada\? So that the rule may not apply to उञ् the form assumed by वेञ् by samprasarana as तन्त्रे उतं = तन्त्रयुतम् ॥ Objection:- उञ् could never have meant the form assumed by वेञ्, for the samprasarana of वेञ् is उ, the ञ् is merely indicatory. Moreover the maxim of lakshana-pratipadokta &c, will prevent the inclusion of this उञ् resulting from vocalisation, when there is a separate Particle उञ् ॥
Answer:- The word पदे is used here for the sake of the subsequent sutras like 8.3.32. So that ङ्मुट् may come before a word beginning with a vowel, and not before a vowel which is an affix. Thus there is no double in न् in परमदण्डिना ॥ This is also a nitya rule, and not optional. Had it been optional, there would have been no necessity of this aphorism, because 8.3.19, would have been enough.,