Before the affix ḍha (eya), the vṛddhi is substituted for the first vowel of the second member of pravāhaṇa but optionally so for the first member (i.e. pra).,
Thus प्रवाहणस्यापत्यं = प्रावाहणेयः or प्रवाहणेयः ॥ The affix ढक् 4.1.123, is added, similarly प्रवाहणेयी भार्याऽस्य = प्रवाहणेयीभार्यः 6.3.39, masculation prohibited)). Or we may translate the sutra as \Before the affix ढ, the vowel of प्र in प्रवाहण optionally gets Vriddhi\, and omit the rest. The masculation will still be prohibited by 6.3.41, (जाते),