The visarga of is or us ending word which is not preceded by any other word is invariably changed to ṣ in a compound when followed by a hard consonant or a labial.,
The words इस् and उस् are understood here. Thus सर्पिष्कुण्डिका, धनुष्कपालम्, सर्पिष्पानम्, धनुष्फलम् ॥
Why do we say when it is not preceded by another word? Observe परमसर्पिः कुण्डिका, परमधनुः कपालम् ॥ The option even of the last sutra does not apply to these examples.
Question: The word सर्पिस् is derived from the root सृप् by adding the Unadi affix इसि (Unadi II. 109), and यजुस् by the Unadi affix उसि (Unadi II. 117), therefore on the maxim प्रत्ययग्रहणे &c, the word इसुसोः would denote the mere forms सर्पिस् and यजुस् and not forms like परमसर्पिस् &c. then what is the necessity of employing the word अनुत्तरपदस्थस्य in the sutra?
The very employment of the word anuttara-pada-sthasya in this sutra, is an indicator (jnapaka), that the restriction of the following maxim does not apply with regard to the affixes इस् and उस् : प्रत्ययग्रहणे यस्मात् स विहित स्तदादेस्तदन्तस्य ग्रहणम् \an affix denotes, whenever it is employed in Grammar, a word-form which begins with that to which that affix has been added and ends with the affix itself\. This maxim not applying, we have परमसर्पिष्करोति or परमसर्पिः करोति by the previous sutra 8.3.44.
Question: Why is not there option in the case of compounds also by the previous sutra?
Answer:- Because the word सामर्थ्य there means व्यपेक्षा, and therefore does not apply to compounds.,