The letter da is the substitute of the final k of an indeclinable when the affix akac (ak) is added.,
The word अव्यय is to be read into this sutra and not सर्वनाम, because no Pronominal ends in क् ॥ Thus धकित्, हिरकुत् and पृथकत् from धिक्, हिरुक् and पृथक् ॥ Thus हिरुक् + अक् = हिर् + अक् + उक् = हिरकुत्, now comes the present rule and the final क् is changed into द्, as हिरकुद् or त् ॥,