For the final long ṝ of a root, there is substituted ir -- uraṇ raparaḥ [[1.1.51]].,
As किरति, गिरति from कॄ and गॄ of the Tudadi class. आस्तीर्णम् विस्तीर्णम् from स्तॄ, the lengthening by 8.2.77. Why do we say of a root? Observe पितॄणाम्, मातॄणाम् ॥
This substitution will apply to Derivative roots also, as चिकीर्षति from कॄ 'to scatter'.,