There is substitution of pī for pyāy when the affixes of the Perfect (liṭ) and the Intensive (yaṅ) follow.,
The anuvritti of the word विभाषा ceases. The phrase प्यायः पी of the last sutra is drawn into this by force of the word च 'and'; thus आपिप्ये, आपिप्याते, आपिप्यिरे ॥
The substitution of पी a subsequent rule, would have debarred reduplication which precedes it in order. The reduplication, however, takes place after the substitution, by force of the maxim of पुनः प्रसङ्ग &c mentioned in 6.1.17. Thus पी + लिट् = पि + पी + त = पि + प्य + ए 6.4.82 the य being substituted) = आपिप्ये with the affix आ. So also in the Intensive, as, आपेपीयते, आपेपीयेते, आपेपीयन्ते ॥,