Short ṛ is optionally substituted for the penultimate ṛ and ṝ of a Causative aṅga (stem) in the reduplicated Aorist.,
This debars the इर् 7.3.101, अर् 7.3.86, and आर् 7.2.114 substitutes. Thus अचिकीर्तत् (VII. I. 101) or अचिकिर्तत् from कॄत् (Churadi III) : अववर्तत् 7.3.86, or अवीवृतत्; अममार्जत् 7.2.114 or अमीमृजत् ॥ Though the इर्, अर् and आर् substitutes are antaranga operations, they are prohibited by the express text of this sutra. The short ऋ is substituted even for a long ॠ : the त् of ऋत् shows that, as in अचीकृतत् 8.2.77. In fact, this ऋ substitute does not take place after the operations of इर्, अर् and आर् substitutions have taken effect, but it is a form which suspends the operation of all those rules.,