The affix aṇ comes after those verbs which end in long ā when an upasarga is in composition and when the word to be formed is feminine.,
This supersedes क्तिन्. Thus प्रदा, प्रधा, उपदा, उपधा, &c.
Vart:- For the purposes of this rule the particles श्रत् and अन्तर् should be treated like upasargas. Thus श्रद्धा 'faith' अन्तर्द्धा 1.4.59.,