The ṣ substitution does not take place in the reduplicated Aorist of the roots 1. stambh 2. siv and 3. saha,
The root स्तम्भ required ष change by 8.3.67, and the roots सिव and सह by 8.3.70. Thus पर्यतस्तम्भत्, अभ्यतस्तम्भत् ॥ सिव :- पर्यसीषिवत्, न्यसीषिवत् ॥ सह :- पर्यसीषहत्, व्यसीषहत् ॥
Vart:- This prohibition refers to the ष change that is caused by an Upasarga and not to the reduplicate change, as we have illustrated in the above example.,