The penultimate vowel in the Causative aṅga (stem) pā 'to drink' is elided in the Aorist and for the vowel of the reduplicate there is substituted long ī,
As पा + णि + अत् = पाय् + इ + अत् 7.3.37 = पाय् + अत् 6.4.51 = प्य् + अत् 7.4.8 = पप्य् + अत् (I. I. 59) = पीप्यत् 7.4.4. Thus अपीप्यत्, अपीप्यताम्, अपीप्यन् ॥ When the penultimate आ is elided, there remains प् which cannot be reduplicated as having no vowel, but the elided आ is considered as sthanivat and thus पा is reduplicated. Sutra 7.4.80 establishes by implication the principle that the substitutes caused by णि are sthanivat. Thus भू + णि = भावि; in reduplicating, this भा will not be reduplicated, but भू, as बुभावि, भा = भू by sthanivad-bhava.,