मातुःपितुर्भ्यामन्यतरस्याम्

Thus मातुःस्वसा or मातुःष्वसा; पितुःस्वसा or पितुःष्वसा ॥ The word मातुर् and पितुर् end in र्, which is changed to visarga (See 8.2.24). In fact, the word पितुर् is so exhibited in the sutra itself, with a र्, the word मातुः therefore, by the rule of साहचर्य is also to be understood as a र् ending word.

Question:- Well, if this be so, when this र् is changed to a visarga, and the visarga changed to स् by 8.3.36, then there would be no ष change ?

Answer:- The ष change would take place both after the visarga-ending or स-ending words, on the maxim एकदेशविकृतस्यानन्यत्वात् \That which has undergone a change in regard to one of its parts, is by no means, in consequence of this change, something else than what it was before the change had taken place\. Therefore, these words are taken to end in र् ॥ For if they were taken to end in स्, then the rule would not apply when they ended in a visarga. If they be supposed to end in a visarga, then the form पितुर्भ्याम् with a र् before भ्याम् in the sutra is hard to explain.

The word समासे is understood here also. Therefore not here मातुः स्वसा when the words are used separately in a sentence.,