This substitution of ais does not take place after 1. idam and 2. adas except when they end in ka,
As एभिः, अमीभिः, but इमकैः, अमुकैः ॥ By 7.2.102, अ is substituted for the final of इदम्, as इद + अ = इद 6.1.97. By 7.2.113, the इद् is elided before भिस्, and the only portion left is अ, which becomes ए according to 7.3.103. For the final of अदस् is similarly by 7.2.102, अ substituted, and अद + अ = अद 6.1.97, and according to our present sutra, the form in Instrumental plural would be अदेभिः, but द् is changed to म् by 8.2.80, and ए to ई by 8.2.81.
The exception made with regard to these forms ending in क, shows the existence of the following maxim : तद्मध्यपतितस्तद्ग्रहणेन गृह्यते \Any term that may be employed in Grammar denotes not merely what is actually denoted by it, but it denotes also whatever word-form may result when something is inserted in that which is actually denoted by it.\
The sutra has not been made as इदमदसोः कात्, for had it been so constructed the rule would have applied to the क ending इदम् and अदस् and to no other क ending pronoun, and we could not get the forms सर्वकैः, विश्वकैः ॥ Moreover such a construction would have made ऐस् applicable to इदम् and अदस् also in their simple states : i. e. we could not have got the forms एभिः or अमीभिः ॥ Therefore the negative construction न अकोः is used in sutra in order to prevent the rule of contrariety.,