रो रि

There is elision of र् when र् follows.
The sutra is रः रि, and not रोः रि ॥ That is रो रि is the form which रः रि and रोः रि will both assume. रः is the Genitive of र्, and रोः would be the Genitive of रु ॥ The sutra is not confined to रु only, but to every र् in general including रु ॥ Thus नीरक्तम्, दूरक्तम्, where it is simple र् of निर् and दुर्; and अग्नी रथः, and इन्दू रथः where it is रु (अग्निः रथः, इन्दुः रथः). The lengthening is by 6.3.111. The word पदस्य is understood here, and the Genitive here has the force of विशेषण i. e. a quality, or avayava-shashthi; i. e. when रेफ is a portion of the pada. Thus a रेफ which is not at the end of a pada is also elided. Had the Genitive been construed as sthana-shashthi, then रेफ would qualify पद, and the rule would mean "र् should be elided before a र् when at the end of a word." See 8.1.16. But we have the elision of the penultimate र् in अजर्घर्र् the second person singular of the Imperfect of the Intensive, and we get the form अजर्घाः ॥ See 8.2.37. So also अपास्पाः from स्पर्ध in Intensive, Imperfect. The reduplicate is lengthened by 7.4.83, the शप् is elided, and सिप् is elided by 6.1.68, the final ध् is changed to द् by 8.2.39, and this द् changed to र् by 8.2.75.