विभाषा लीयतेः

There is optionally the substitution of आ for the final of ली 'to adhere', in the धातुपाठ , when the affix ल्यप् follows or such an affix as would demand this substitution of the diphthong.
The final of ली will take guna substitution before शित् affixes, and will become ले, this incipient diphthong ए is changed to आ by this rule. The same is the case with all the roots subsequently taught, thus खिद् will be खेद्, and then ए changed to आ, गुर् = गोर् and ओ changed to आ &c. Therefore we have employed the anuvritti of एच् 'diphthong' in all sutras. The words ल्याप and आदेच उपदेश are understood here. The roots ली belonging to Divadi and Kriyadi are both included here. Thus विलाता, विलातुम्, विलातव्यम्, विलाय, विलेता विलेतुम्, विलेतव्यम् and विलीय ॥

Vart:- The आ substitution does not take place when the affix अच् 3.3.56, 3.1.134 and खल् 3.3.126 come after नि, मि, मी and ली : as, ईषन्निमयः, निमयो वर्त्तते । ईषत्प्रमयः, प्रमयो वर्त्तते । ईषद् विलयः, विलयो वर्त्तते ॥

The ली invariably takes आ substitution when the sense is that of 'showing respect', 'deceiving' or 'insulting'. As कस्त्वामुल्लापयते, श्येनो वर्त्तिकामुल्लापयते ॥ The option allowed by this aphorism is a restricted option (vyavasthita-vibhasha) The substitution of आ for the final of ई is optional when the sense of the root is not that of 'showing respect', 'subduing' or deceiving'. But when it has any one of these three senses, the substitution is compulsory. See (l. 1.70).