a is substituted for the vowel of the reduplicate of bhū 'to be' in the liṭ (Perfect Tense).,
As बभूव, बभूवतुः, बभूवुः, बभूवे ॥ Why do we say 'in the Perfect'? Observe बभूषति and बोभूयते ॥
The word भवतेः is shown in the sutra in the Active Voice, therefore the rule will not apply to Passive and Reflexive forms; as अनुबुभूवे कम्बलो देवदत्तेन ॥ This opinion of Kasika, however, is not endorsed by later Grammarians.,