इदुदुपधस्य चाप्रत्ययस्य

ष् is substituted before a hard guttural or a labial for the विसर्ग which is preceded इ or उ and not part of an affix.
This applies to the visarga of निर्, दुर्, बहिर्, अविस्, चतुर् and प्रादुस् ॥ Thus निष्कृतम्, निष्पीतम्, दुष्कृतम्, दुष्पीतम्, बहिस्, बहिष्कृतम्, बहिष्पीतम्, आविस्, आविष्कृतम्, आविष्पीतम्, चतुष्कृतम्, चतुष्कपालम्, चतुष्कन्दलम्, चतुष्कलम्, प्रादुष्कृतम्, प्रादुष्पीतम् ॥

Why do we say 'when not belonging to an affix'? Observe अग्निः करोति, वायुः करोति ॥ How do you explain मातुः करोति, पितुः करोति? For here in पितुः &c, the स् of the affix is elided by 8.2.24, and the र् of पितुर् + स् is changed to visarga; this is not the visarga of an affix, and ought to be changed to ष् ॥

Answer:- The inclusion of the word भ्रातुष्पुत्रः in Kaskadi class 8.3.48 indicates by implication, that ष change does not take place of this visarga in पितुः, मातुः &c: the only exception being भ्रातुः ॥ The reason of this may be that the visarga here does not follow a simple उ, but an ekadesa उ 6.1.111.

Vart:- Prohibition must be stated in the case of पुम् and मुहुस्; as पुंस्कामा, मुहुस्कामा ॥

Vart:- वृद्धिभूतानां षत्वं वक्तव्यम् । The visarga is changed to ष even when इ or उ are vriddhied; as नैष्कुल्यम्, दौष्कुल्यम् ॥

Vart:- प्लुतानां तादौ च ॥ The visarga is changed to ष even when इ or उ are pluta, and then before dentals as well as gutturals and labials. Thus नि३ष्कुलम्, दु३ष्कुलम् 8.2.86 for pluta)). दु३ष्पुरुषः, वही३ष्टरः ॥

These last two vartikas may be dispensed with, because Vriddhi and Pluta are Bahiranga change, and so the visarga will be changed to ष in these cases also by the sutra itself; except so far as त is concerned.