नेन्द्रस्य परस्य

But the वृद्धि of the first vowel of इन्द्र when it stands as the second member of a द्वन्द समास , does not take place before a तद्धित affix having an indicatory ञ् , ण् or क् ।
As सौमेन्द्रः, आग्नेन्द्रः ॥ Why "when it stands as the subsequent member"? Observe ऐन्द्राग्न मेकादशकपालं चरुं निर्वपेत् ॥ There are two vowels in the word इन्द्र, and when a Taddhita affix is added, then one of these i. e. the अ or the last vowel is elided by 6.4.148, and the other (i. e. the इ, coalesces with the last vowel of the first term, as सोम + इन्द्र + अण् = सोम + इन्द्र् + अ = सोमे + न्द्र् + अ ॥ Now, no vowel is left of इन्द्र when it gets the form न्द्र्, so what is the necessity of the present prohibitory rule? This prohibition indicates the existence of the following maxim : बहिरङ्गमपि पूर्वोत्तरपदयोः पूर्वं कार्यं भवति, पश्चादेकादेशः or in other words पूर्वोत्तरनिमित्तकार्यात् पूर्वमन्तरङ्गोऽप्येकादेशो न ॥ "The substitution of one vowel for the final of the first and the initial of the second member of a compound does, even when it is antaranga, not take place previously to an operation which concerns the first or the second member of the compound". It is on this maxim that the forms पूर्वशुकामशम 7.3.14 &c are constructed, otherwise उ being the first vowel of the second member (इ of इषु having merged in पूर्व), would have been vriddhied.