णौ च संश्चङोः

There is optionally the vocalisation of the semi-vowel, in the causative of श्वि , when followed by the Desiderative सन् and the Aorist चङ् affix.
The phrase विभाषा श्वेः is understood here. In forming the Desiderative and the Reduplicated Aorist forms of the Causative of श्वि, there is optional vocalisation. Thus शुशावयिषति or शिश्वाययिषति ॥ So also in the Reduplicated Aorist चङ्, as अशृशवत् or अशिश्वयत् ॥ By the maxim संप्रसारणं संप्रसारणाश्रयं च बलीयो भवति (see sutra 6.1.17), the antaranga substitution of Vriddhi &c is superseded by the samprasarana and the subsidiary operations relating to it here. The vriddhi and the substitution of आव take place after the samprasarana has taken place. The sutra 7.4.80 teaching the substitution of ई for the उ of the reduplicate, when सन् follows, shows by implication that the substitution caused by णि is sthanivat though it itself does not cause reduplication 1.1.59. Thus श्वि + णि = श्वै + इ = श्वाय् + इ = श्वायि ॥ In reduplicating श्वायि we cannot form श्वाश्वाय, but must consider the substitute equal to the original इ of श्वि and must reduplicate it. We thus get शिश्वाययिषति ॥ In the case of vocalisation, the rules of Vriddhi &c are postponed. Thus श्वि + णि + सन् 6.1.108 = शु + णि + सन् = शौ + इ + सन् = शाव् + इ + इट् + सन् = शावे + इट् + सन् = शावयि + सन् ॥ Here we require reduplication. Here the substitute आव् in शाव् though not caused by सन्, is treated sthanivat under 1.1.59 to उ, which is carried in reduplication. Similarly the Aorist. श्वि + णि + चङ् = शु + इ + अ = शु + शु + इ + अ = शु + शौ + इ + अ = शु + शाव् + इ + अ = शु + शव् + इ + अ 7.4.1 = शु + शव् + ० + अ 6.4.51 = अशूशवत् 7.4.94