A long vowel is substituted in the liṭ (Perfect Tense) for the initial a of the reduplicate.,
This debars the single substitute of the form of the subsequent ordained by (VI. I. 97). Thus आट, आटतुः, आटुः ॥ Why do we say 'the initial'? Observe पपाच and पपाठ ॥ Thus अट् + अतुः = अट् अट् + अतुः = अ अट् + अतुः = आ अट् ॥,