pī is optionally the substitute of the root pyāy 'to increase' before the niṣṭhā affixes.,
The root ओप्यायी वृद्धौ belongs to Bhvadi class. The indicatory आ shows that the affixes त and तवत् are changed into न and नवत् 8.2.45. Thus पीनं मुखम्, पीनौ बाहू, पीनसुरः ॥ The option here is also a regulated option (vyavasthita-vibhasha). The substitution takes place invariably when the root is without upasarga, and never when it is preceded by an upasarga. Thus प्रप्यानः आप्यानश्चन्द्रगाः ॥ The substitution, however, must take place when the preposition आङ् precedes the root, and the words अन्धुः and ऊधस् are in composition : as आपीनोन्धुः, आपीनमूधः ॥,