i is optionally substituted for the final u of a reduplicate of 1. sru 2. śru 3. dru 4. pru 5. plu and 6. cyu when the semi-vowel is followed by a a or ā in the Desiderative.,
Thus सिस्रावयिषति or सुस्रावयिषति; शिश्रावयिषति or शुश्रावयिषति; दिद्रावयिषति or दुद्रावयिषति; पिप्रावयिषति or पुप्रावयिषति; पिप्लावयिषति or पुप्लावयिषति; चिच्यावयिषति or चुच्यावयिषति ॥ All these are Desideratives of the Causatives of the above roots. Here though a letter like स्, श्, द् &c, intervenes between the semi-vowel and the preceding उ of the reduplicate, yet the substitution takes place owing to the express text of this sutra. In the preceding aphorism, the semi-vowel followed immediately after the उ of the reduplicate. The option of the present sutra is, therefore, an aprapta-vibhasha. The word अपरे is understood here also: therefore not in सुस्रूषति, शुश्रूषति ॥,