Of the causative verb svapi 'to cause one to sleep', there is vocalisation of the semi-vowel, when the affix caṅ of the Reduplicated Aorist follows.,
Thus the Aorist of स्वपि is असूषुपत्, असूषुपताम्, असूषुपन् ॥ The vocalisation takes place before reduplication, then there is guna of the penultimate short vowel, then this is again shortened by 7.4.1, then there is reduplication, and then lengthening of the vowel of the reduplicate by 7.4.94. Thus स्वापि + चङ् = सुपि + चङ् = सुप् 6.4.51 + चङ् 7.3.86 = सोप् + चङ् = सुप् + चङ् 7.4.1 = सु + सुप् + चङ् 6.1.11 = सूषुपत् 7.4.94 which with the augment अ becomes असूषुपत् ॥
Why do we say 'when चङ् follows'? Observe स्वाप्यते, स्वापितः ॥ The anuvritti of किति has ceased, that of ङिति however is here.,